diff -r 000000000000 -r 7f656887cf89 libraries/spcre/libpcre/pcre/doc/pcre.txt --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/libraries/spcre/libpcre/pcre/doc/pcre.txt Wed Jun 23 15:52:26 2010 +0100 @@ -0,0 +1,6616 @@ +----------------------------------------------------------------------------- +This file contains a concatenation of the PCRE man pages, converted to plain +text format for ease of searching with a text editor, or for use on systems +that do not have a man page processor. The small individual files that give +synopses of each function in the library have not been included. There are +separate text files for the pcregrep and pcretest commands. +----------------------------------------------------------------------------- + + +PCRE(3) PCRE(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +INTRODUCTION + + The PCRE library is a set of functions that implement regular expres- + sion pattern matching using the same syntax and semantics as Perl, with + just a few differences. Certain features that appeared in Python and + PCRE before they appeared in Perl are also available using the Python + syntax. There is also some support for certain .NET and Oniguruma syn- + tax items, and there is an option for requesting some minor changes + that give better JavaScript compatibility. + + The current implementation of PCRE (release 7.x) corresponds approxi- + mately with Perl 5.10, including support for UTF-8 encoded strings and + Unicode general category properties. However, UTF-8 and Unicode support + has to be explicitly enabled; it is not the default. The Unicode tables + correspond to Unicode release 5.0.0. + + In addition to the Perl-compatible matching function, PCRE contains an + alternative matching function that matches the same compiled patterns + in a different way. In certain circumstances, the alternative function + has some advantages. For a discussion of the two matching algorithms, + see the pcrematching page. + + PCRE is written in C and released as a C library. A number of people + have written wrappers and interfaces of various kinds. In particular, + Google Inc. have provided a comprehensive C++ wrapper. This is now + included as part of the PCRE distribution. The pcrecpp page has details + of this interface. Other people's contributions can be found in the + Contrib directory at the primary FTP site, which is: + + ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre + + Details of exactly which Perl regular expression features are and are + not supported by PCRE are given in separate documents. See the pcrepat- + tern and pcrecompat pages. There is a syntax summary in the pcresyntax + page. + + Some features of PCRE can be included, excluded, or changed when the + library is built. The pcre_config() function makes it possible for a + client to discover which features are available. The features them- + selves are described in the pcrebuild page. Documentation about build- + ing PCRE for various operating systems can be found in the README file + in the source distribution. + + The library contains a number of undocumented internal functions and + data tables that are used by more than one of the exported external + functions, but which are not intended for use by external callers. + Their names all begin with "_pcre_", which hopefully will not provoke + any name clashes. In some environments, it is possible to control which + external symbols are exported when a shared library is built, and in + these cases the undocumented symbols are not exported. + + +USER DOCUMENTATION + + The user documentation for PCRE comprises a number of different sec- + tions. In the "man" format, each of these is a separate "man page". In + the HTML format, each is a separate page, linked from the index page. + In the plain text format, all the sections are concatenated, for ease + of searching. The sections are as follows: + + pcre this document + pcre-config show PCRE installation configuration information + pcreapi details of PCRE's native C API + pcrebuild options for building PCRE + pcrecallout details of the callout feature + pcrecompat discussion of Perl compatibility + pcrecpp details of the C++ wrapper + pcregrep description of the pcregrep command + pcrematching discussion of the two matching algorithms + pcrepartial details of the partial matching facility + pcrepattern syntax and semantics of supported + regular expressions + pcresyntax quick syntax reference + pcreperform discussion of performance issues + pcreposix the POSIX-compatible C API + pcreprecompile details of saving and re-using precompiled patterns + pcresample discussion of the sample program + pcrestack discussion of stack usage + pcretest description of the pcretest testing command + + In addition, in the "man" and HTML formats, there is a short page for + each C library function, listing its arguments and results. + + +LIMITATIONS + + There are some size limitations in PCRE but it is hoped that they will + never in practice be relevant. + + The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE + is compiled with the default internal linkage size of 2. If you want to + process regular expressions that are truly enormous, you can compile + PCRE with an internal linkage size of 3 or 4 (see the README file in + the source distribution and the pcrebuild documentation for details). + In these cases the limit is substantially larger. However, the speed + of execution is slower. + + All values in repeating quantifiers must be less than 65536. + + There is no limit to the number of parenthesized subpatterns, but there + can be no more than 65535 capturing subpatterns. + + The maximum length of name for a named subpattern is 32 characters, and + the maximum number of named subpatterns is 10000. + + The maximum length of a subject string is the largest positive number + that an integer variable can hold. However, when using the traditional + matching function, PCRE uses recursion to handle subpatterns and indef- + inite repetition. This means that the available stack space may limit + the size of a subject string that can be processed by certain patterns. + For a discussion of stack issues, see the pcrestack documentation. + + +UTF-8 AND UNICODE PROPERTY SUPPORT + + From release 3.3, PCRE has had some support for character strings + encoded in the UTF-8 format. For release 4.0 this was greatly extended + to cover most common requirements, and in release 5.0 additional sup- + port for Unicode general category properties was added. + + In order process UTF-8 strings, you must build PCRE to include UTF-8 + support in the code, and, in addition, you must call pcre_compile() + with the PCRE_UTF8 option flag. When you do this, both the pattern and + any subject strings that are matched against it are treated as UTF-8 + strings instead of just strings of bytes. + + If you compile PCRE with UTF-8 support, but do not use it at run time, + the library will be a bit bigger, but the additional run time overhead + is limited to testing the PCRE_UTF8 flag occasionally, so should not be + very big. + + If PCRE is built with Unicode character property support (which implies + UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup- + ported. The available properties that can be tested are limited to the + general category properties such as Lu for an upper case letter or Nd + for a decimal number, the Unicode script names such as Arabic or Han, + and the derived properties Any and L&. A full list is given in the + pcrepattern documentation. Only the short names for properties are sup- + ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let- + ter}, is not supported. Furthermore, in Perl, many properties may + optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE + does not support this. + + Validity of UTF-8 strings + + When you set the PCRE_UTF8 flag, the strings passed as patterns and + subjects are (by default) checked for validity on entry to the relevant + functions. From release 7.3 of PCRE, the check is according the rules + of RFC 3629, which are themselves derived from the Unicode specifica- + tion. Earlier releases of PCRE followed the rules of RFC 2279, which + allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current + check allows only values in the range U+0 to U+10FFFF, excluding U+D800 + to U+DFFF. + + The excluded code points are the "Low Surrogate Area" of Unicode, of + which the Unicode Standard says this: "The Low Surrogate Area does not + contain any character assignments, consequently no character code + charts or namelists are provided for this area. Surrogates are reserved + for use with UTF-16 and then must be used in pairs." The code points + that are encoded by UTF-16 pairs are available as independent code + points in the UTF-8 encoding. (In other words, the whole surrogate + thing is a fudge for UTF-16 which unfortunately messes up UTF-8.) + + If an invalid UTF-8 string is passed to PCRE, an error return + (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know + that your strings are valid, and therefore want to skip these checks in + order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at + compile time or at run time, PCRE assumes that the pattern or subject + it is given (respectively) contains only valid UTF-8 codes. In this + case, it does not diagnose an invalid UTF-8 string. + + If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, + what happens depends on why the string is invalid. If the string con- + forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a + string of characters in the range 0 to 0x7FFFFFFF. In other words, + apart from the initial validity test, PCRE (when in UTF-8 mode) handles + strings according to the more liberal rules of RFC 2279. However, if + the string does not even conform to RFC 2279, the result is undefined. + Your program may crash. + + If you want to process strings of values in the full range 0 to + 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can + set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in + this situation, you will have to apply your own validity check. + + General comments about UTF-8 mode + + 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a + two-byte UTF-8 character if the value is greater than 127. + + 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 + characters for values greater than \177. + + 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi- + vidual bytes, for example: \x{100}{3}. + + 4. The dot metacharacter matches one UTF-8 character instead of a sin- + gle byte. + + 5. The escape sequence \C can be used to match a single byte in UTF-8 + mode, but its use can lead to some strange effects. This facility is + not available in the alternative matching function, pcre_dfa_exec(). + + 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly + test characters of any code value, but the characters that PCRE recog- + nizes as digits, spaces, or word characters remain the same set as + before, all with values less than 256. This remains true even when PCRE + includes Unicode property support, because to do otherwise would slow + down PCRE in many common cases. If you really want to test for a wider + sense of, say, "digit", you must use Unicode property tests such as + \p{Nd}. + + 7. Similarly, characters that match the POSIX named character classes + are all low-valued characters. + + 8. However, the Perl 5.10 horizontal and vertical whitespace matching + escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char- + acters. + + 9. Case-insensitive matching applies only to characters whose values + are less than 128, unless PCRE is built with Unicode property support. + Even when Unicode property support is available, PCRE still uses its + own character tables when checking the case of low-valued characters, + so as not to degrade performance. The Unicode property information is + used only for characters with higher values. Even when Unicode property + support is available, PCRE supports case-insensitive matching only when + there is a one-to-one mapping between a letter's cases. There are a + small number of many-to-one mappings in Unicode; these are not sup- + ported by PCRE. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + Putting an actual email address here seems to have been a spam magnet, + so I've taken it away. If you want to email me, use my two initials, + followed by the two digits 10, at the domain cam.ac.uk. + + +REVISION + + Last updated: 12 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREBUILD(3) PCREBUILD(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE BUILD-TIME OPTIONS + + This document describes the optional features of PCRE that can be + selected when the library is compiled. It assumes use of the configure + script, where the optional features are selected or deselected by pro- + viding options to configure before running the make command. However, + the same options can be selected in both Unix-like and non-Unix-like + environments using the GUI facility of CMakeSetup if you are using + CMake instead of configure to build PCRE. + + The complete list of options for configure (which includes the standard + ones such as the selection of the installation directory) can be + obtained by running + + ./configure --help + + The following sections include descriptions of options whose names + begin with --enable or --disable. These settings specify changes to the + defaults for the configure command. Because of the way that configure + works, --enable and --disable always come in pairs, so the complemen- + tary option always exists as well, but as it specifies the default, it + is not described. + + +C++ SUPPORT + + By default, the configure script will search for a C++ compiler and C++ + header files. If it finds them, it automatically builds the C++ wrapper + library for PCRE. You can disable this by adding + + --disable-cpp + + to the configure command. + + +UTF-8 SUPPORT + + To build PCRE with support for UTF-8 character strings, add + + --enable-utf8 + + to the configure command. Of itself, this does not make PCRE treat + strings as UTF-8. As well as compiling PCRE with this option, you also + have have to set the PCRE_UTF8 option when you call the pcre_compile() + function. + + +UNICODE CHARACTER PROPERTY SUPPORT + + UTF-8 support allows PCRE to process character values greater than 255 + in the strings that it handles. On its own, however, it does not pro- + vide any facilities for accessing the properties of such characters. If + you want to be able to use the pattern escapes \P, \p, and \X, which + refer to Unicode character properties, you must add + + --enable-unicode-properties + + to the configure command. This implies UTF-8 support, even if you have + not explicitly requested it. + + Including Unicode property support adds around 30K of tables to the + PCRE library. Only the general category properties such as Lu and Nd + are supported. Details are given in the pcrepattern documentation. + + +CODE VALUE OF NEWLINE + + By default, PCRE interprets character 10 (linefeed, LF) as indicating + the end of a line. This is the normal newline character on Unix-like + systems. You can compile PCRE to use character 13 (carriage return, CR) + instead, by adding + + --enable-newline-is-cr + + to the configure command. There is also a --enable-newline-is-lf + option, which explicitly specifies linefeed as the newline character. + + Alternatively, you can specify that line endings are to be indicated by + the two character sequence CRLF. If you want this, add + + --enable-newline-is-crlf + + to the configure command. There is a fourth option, specified by + + --enable-newline-is-anycrlf + + which causes PCRE to recognize any of the three sequences CR, LF, or + CRLF as indicating a line ending. Finally, a fifth option, specified by + + --enable-newline-is-any + + causes PCRE to recognize any Unicode newline sequence. + + Whatever line ending convention is selected when PCRE is built can be + overridden when the library functions are called. At build time it is + conventional to use the standard for your operating system. + + +WHAT \R MATCHES + + By default, the sequence \R in a pattern matches any Unicode newline + sequence, whatever has been selected as the line ending sequence. If + you specify + + --enable-bsr-anycrlf + + the default is changed so that \R matches only CR, LF, or CRLF. What- + ever is selected when PCRE is built can be overridden when the library + functions are called. + + +BUILDING SHARED AND STATIC LIBRARIES + + The PCRE building process uses libtool to build both shared and static + Unix libraries by default. You can suppress one of these by adding one + of + + --disable-shared + --disable-static + + to the configure command, as required. + + +POSIX MALLOC USAGE + + When PCRE is called through the POSIX interface (see the pcreposix doc- + umentation), additional working storage is required for holding the + pointers to capturing substrings, because PCRE requires three integers + per substring, whereas the POSIX interface provides only two. If the + number of expected substrings is small, the wrapper function uses space + on the stack, because this is faster than using malloc() for each call. + The default threshold above which the stack is no longer used is 10; it + can be changed by adding a setting such as + + --with-posix-malloc-threshold=20 + + to the configure command. + + +HANDLING VERY LARGE PATTERNS + + Within a compiled pattern, offset values are used to point from one + part to another (for example, from an opening parenthesis to an alter- + nation metacharacter). By default, two-byte values are used for these + offsets, leading to a maximum size for a compiled pattern of around + 64K. This is sufficient to handle all but the most gigantic patterns. + Nevertheless, some people do want to process enormous patterns, so it + is possible to compile PCRE to use three-byte or four-byte offsets by + adding a setting such as + + --with-link-size=3 + + to the configure command. The value given must be 2, 3, or 4. Using + longer offsets slows down the operation of PCRE because it has to load + additional bytes when handling them. + + +AVOIDING EXCESSIVE STACK USAGE + + When matching with the pcre_exec() function, PCRE implements backtrack- + ing by making recursive calls to an internal function called match(). + In environments where the size of the stack is limited, this can se- + verely limit PCRE's operation. (The Unix environment does not usually + suffer from this problem, but it may sometimes be necessary to increase + the maximum stack size. There is a discussion in the pcrestack docu- + mentation.) An alternative approach to recursion that uses memory from + the heap to remember data, instead of using recursive function calls, + has been implemented to work round the problem of limited stack size. + If you want to build a version of PCRE that works this way, add + + --disable-stack-for-recursion + + to the configure command. With this configuration, PCRE will use the + pcre_stack_malloc and pcre_stack_free variables to call memory manage- + ment functions. By default these point to malloc() and free(), but you + can replace the pointers so that your own functions are used. + + Separate functions are provided rather than using pcre_malloc and + pcre_free because the usage is very predictable: the block sizes + requested are always the same, and the blocks are always freed in + reverse order. A calling program might be able to implement optimized + functions that perform better than malloc() and free(). PCRE runs + noticeably more slowly when built in this way. This option affects only + the pcre_exec() function; it is not relevant for the the + pcre_dfa_exec() function. + + +LIMITING PCRE RESOURCE USAGE + + Internally, PCRE has a function called match(), which it calls repeat- + edly (sometimes recursively) when matching a pattern with the + pcre_exec() function. By controlling the maximum number of times this + function may be called during a single matching operation, a limit can + be placed on the resources used by a single call to pcre_exec(). The + limit can be changed at run time, as described in the pcreapi documen- + tation. The default is 10 million, but this can be changed by adding a + setting such as + + --with-match-limit=500000 + + to the configure command. This setting has no effect on the + pcre_dfa_exec() matching function. + + In some environments it is desirable to limit the depth of recursive + calls of match() more strictly than the total number of calls, in order + to restrict the maximum amount of stack (or heap, if --disable-stack- + for-recursion is specified) that is used. A second limit controls this; + it defaults to the value that is set for --with-match-limit, which + imposes no additional constraints. However, you can set a lower limit + by adding, for example, + + --with-match-limit-recursion=10000 + + to the configure command. This value can also be overridden at run + time. + + +CREATING CHARACTER TABLES AT BUILD TIME + + PCRE uses fixed tables for processing characters whose code values are + less than 256. By default, PCRE is built with a set of tables that are + distributed in the file pcre_chartables.c.dist. These tables are for + ASCII codes only. If you add + + --enable-rebuild-chartables + + to the configure command, the distributed tables are no longer used. + Instead, a program called dftables is compiled and run. This outputs + the source for new set of tables, created in the default locale of your + C runtime system. (This method of replacing the tables does not work if + you are cross compiling, because dftables is run on the local host. If + you need to create alternative tables when cross compiling, you will + have to do so "by hand".) + + +USING EBCDIC CODE + + PCRE assumes by default that it will run in an environment where the + character code is ASCII (or Unicode, which is a superset of ASCII). + This is the case for most computer operating systems. PCRE can, how- + ever, be compiled to run in an EBCDIC environment by adding + + --enable-ebcdic + + to the configure command. This setting implies --enable-rebuild-charta- + bles. You should only use it if you know that you are in an EBCDIC + environment (for example, an IBM mainframe operating system). + + +PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT + + By default, pcregrep reads all files as plain text. You can build it so + that it recognizes files whose names end in .gz or .bz2, and reads them + with libz or libbz2, respectively, by adding one or both of + + --enable-pcregrep-libz + --enable-pcregrep-libbz2 + + to the configure command. These options naturally require that the rel- + evant libraries are installed on your system. Configuration will fail + if they are not. + + +PCRETEST OPTION FOR LIBREADLINE SUPPORT + + If you add + + --enable-pcretest-libreadline + + to the configure command, pcretest is linked with the libreadline + library, and when its input is from a terminal, it reads it using the + readline() function. This provides line-editing and history facilities. + Note that libreadline is GPL-licenced, so if you distribute a binary of + pcretest linked in this way, there may be licensing issues. + + Setting this option causes the -lreadline option to be added to the + pcretest build. In many operating environments with a sytem-installed + libreadline this is sufficient. However, in some environments (e.g. if + an unmodified distribution version of readline is in use), some extra + configuration may be necessary. The INSTALL file for libreadline says + this: + + "Readline uses the termcap functions, but does not link with the + termcap or curses library itself, allowing applications which link + with readline the to choose an appropriate library." + + If your environment has not been set up so that an appropriate library + is automatically included, you may need to add something like + + LIBS="-ncurses" + + immediately before the configure command. + + +SEE ALSO + + pcreapi(3), pcre_config(3). + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 13 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREMATCHING(3) PCREMATCHING(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE MATCHING ALGORITHMS + + This document describes the two different algorithms that are available + in PCRE for matching a compiled regular expression against a given sub- + ject string. The "standard" algorithm is the one provided by the + pcre_exec() function. This works in the same was as Perl's matching + function, and provides a Perl-compatible matching operation. + + An alternative algorithm is provided by the pcre_dfa_exec() function; + this operates in a different way, and is not Perl-compatible. It has + advantages and disadvantages compared with the standard algorithm, and + these are described below. + + When there is only one possible way in which a given subject string can + match a pattern, the two algorithms give the same answer. A difference + arises, however, when there are multiple possibilities. For example, if + the pattern + + ^<.*> + + is matched against the string + + + + there are three possible answers. The standard algorithm finds only one + of them, whereas the alternative algorithm finds all three. + + +REGULAR EXPRESSIONS AS TREES + + The set of strings that are matched by a regular expression can be rep- + resented as a tree structure. An unlimited repetition in the pattern + makes the tree of infinite size, but it is still a tree. Matching the + pattern to a given subject string (from a given starting point) can be + thought of as a search of the tree. There are two ways to search a + tree: depth-first and breadth-first, and these correspond to the two + matching algorithms provided by PCRE. + + +THE STANDARD MATCHING ALGORITHM + + In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- + sions", the standard algorithm is an "NFA algorithm". It conducts a + depth-first search of the pattern tree. That is, it proceeds along a + single path through the tree, checking that the subject matches what is + required. When there is a mismatch, the algorithm tries any alterna- + tives at the current point, and if they all fail, it backs up to the + previous branch point in the tree, and tries the next alternative + branch at that level. This often involves backing up (moving to the + left) in the subject string as well. The order in which repetition + branches are tried is controlled by the greedy or ungreedy nature of + the quantifier. + + If a leaf node is reached, a matching string has been found, and at + that point the algorithm stops. Thus, if there is more than one possi- + ble match, this algorithm returns the first one that it finds. Whether + this is the shortest, the longest, or some intermediate length depends + on the way the greedy and ungreedy repetition quantifiers are specified + in the pattern. + + Because it ends up with a single path through the tree, it is rela- + tively straightforward for this algorithm to keep track of the sub- + strings that are matched by portions of the pattern in parentheses. + This provides support for capturing parentheses and back references. + + +THE ALTERNATIVE MATCHING ALGORITHM + + This algorithm conducts a breadth-first search of the tree. Starting + from the first matching point in the subject, it scans the subject + string from left to right, once, character by character, and as it does + this, it remembers all the paths through the tree that represent valid + matches. In Friedl's terminology, this is a kind of "DFA algorithm", + though it is not implemented as a traditional finite state machine (it + keeps multiple states active simultaneously). + + The scan continues until either the end of the subject is reached, or + there are no more unterminated paths. At this point, terminated paths + represent the different matching possibilities (if there are none, the + match has failed). Thus, if there is more than one possible match, + this algorithm finds all of them, and in particular, it finds the long- + est. In PCRE, there is an option to stop the algorithm after the first + match (which is necessarily the shortest) has been found. + + Note that all the matches that are found start at the same point in the + subject. If the pattern + + cat(er(pillar)?) + + is matched against the string "the caterpillar catchment", the result + will be the three strings "cat", "cater", and "caterpillar" that start + at the fourth character of the subject. The algorithm does not automat- + ically move on to find matches that start at later positions. + + There are a number of features of PCRE regular expressions that are not + supported by the alternative matching algorithm. They are as follows: + + 1. Because the algorithm finds all possible matches, the greedy or + ungreedy nature of repetition quantifiers is not relevant. Greedy and + ungreedy quantifiers are treated in exactly the same way. However, pos- + sessive quantifiers can make a difference when what follows could also + match what is quantified, for example in a pattern like this: + + ^a++\w! + + This pattern matches "aaab!" but not "aaa!", which would be matched by + a non-possessive quantifier. Similarly, if an atomic group is present, + it is matched as if it were a standalone pattern at the current point, + and the longest match is then "locked in" for the rest of the overall + pattern. + + 2. When dealing with multiple paths through the tree simultaneously, it + is not straightforward to keep track of captured substrings for the + different matching possibilities, and PCRE's implementation of this + algorithm does not attempt to do this. This means that no captured sub- + strings are available. + + 3. Because no substrings are captured, back references within the pat- + tern are not supported, and cause errors if encountered. + + 4. For the same reason, conditional expressions that use a backrefer- + ence as the condition or test for a specific group recursion are not + supported. + + 5. Because many paths through the tree may be active, the \K escape + sequence, which resets the start of the match when encountered (but may + be on some paths and not on others), is not supported. It causes an + error if encountered. + + 6. Callouts are supported, but the value of the capture_top field is + always 1, and the value of the capture_last field is always -1. + + 7. The \C escape sequence, which (in the standard algorithm) matches a + single byte, even in UTF-8 mode, is not supported because the alterna- + tive algorithm moves through the subject string one character at a + time, for all active paths through the tree. + + 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) + are not supported. (*FAIL) is supported, and behaves like a failing + negative assertion. + + +ADVANTAGES OF THE ALTERNATIVE ALGORITHM + + Using the alternative matching algorithm provides the following advan- + tages: + + 1. All possible matches (at a single point in the subject) are automat- + ically found, and in particular, the longest match is found. To find + more than one match using the standard algorithm, you have to do kludgy + things with callouts. + + 2. There is much better support for partial matching. The restrictions + on the content of the pattern that apply when using the standard algo- + rithm for partial matching do not apply to the alternative algorithm. + For non-anchored patterns, the starting position of a partial match is + available. + + 3. Because the alternative algorithm scans the subject string just + once, and never needs to backtrack, it is possible to pass very long + subject strings to the matching function in several pieces, checking + for partial matching each time. + + +DISADVANTAGES OF THE ALTERNATIVE ALGORITHM + + The alternative algorithm suffers from a number of disadvantages: + + 1. It is substantially slower than the standard algorithm. This is + partly because it has to search for all possible matches, but is also + because it is less susceptible to optimization. + + 2. Capturing parentheses and back references are not supported. + + 3. Although atomic groups are supported, their use does not provide the + performance advantage that it does for the standard algorithm. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 19 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREAPI(3) PCREAPI(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE NATIVE API + + #include + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre *pcre_compile2(const char *pattern, int options, + int *errorcodeptr, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre_extra *pcre_study(const pcre *code, int options, + const char **errptr); + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize, + int *workspace, int wscount); + + int pcre_copy_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + char *buffer, int buffersize); + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + const char **stringptr); + + int pcre_get_stringnumber(const pcre *code, + const char *name); + + int pcre_get_stringtable_entries(const pcre *code, + const char *name, char **first, char **last); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + void pcre_free_substring(const char *stringptr); + + void pcre_free_substring_list(const char **stringptr); + + const unsigned char *pcre_maketables(void); + + int pcre_fullinfo(const pcre *code, const pcre_extra *extra, + int what, void *where); + + int pcre_info(const pcre *code, int *optptr, int *firstcharptr); + + int pcre_refcount(pcre *code, int adjust); + + int pcre_config(int what, void *where); + + char *pcre_version(void); + + void *(*pcre_malloc)(size_t); + + void (*pcre_free)(void *); + + void *(*pcre_stack_malloc)(size_t); + + void (*pcre_stack_free)(void *); + + int (*pcre_callout)(pcre_callout_block *); + + +PCRE API OVERVIEW + + PCRE has its own native API, which is described in this document. There + are also some wrapper functions that correspond to the POSIX regular + expression API. These are described in the pcreposix documentation. + Both of these APIs define a set of C function calls. A C++ wrapper is + distributed with PCRE. It is documented in the pcrecpp page. + + The native API C function prototypes are defined in the header file + pcre.h, and on Unix systems the library itself is called libpcre. It + can normally be accessed by adding -lpcre to the command for linking an + application that uses PCRE. The header file defines the macros + PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num- + bers for the library. Applications can use these to include support + for different releases of PCRE. + + The functions pcre_compile(), pcre_compile2(), pcre_study(), and + pcre_exec() are used for compiling and matching regular expressions in + a Perl-compatible manner. A sample program that demonstrates the sim- + plest way of using them is provided in the file called pcredemo.c in + the source distribution. The pcresample documentation describes how to + compile and run it. + + A second matching function, pcre_dfa_exec(), which is not Perl-compati- + ble, is also provided. This uses a different algorithm for the match- + ing. The alternative algorithm finds all possible matches (at a given + point in the subject), and scans the subject just once. However, this + algorithm does not return captured substrings. A description of the two + matching algorithms and their advantages and disadvantages is given in + the pcrematching documentation. + + In addition to the main compiling and matching functions, there are + convenience functions for extracting captured substrings from a subject + string that is matched by pcre_exec(). They are: + + pcre_copy_substring() + pcre_copy_named_substring() + pcre_get_substring() + pcre_get_named_substring() + pcre_get_substring_list() + pcre_get_stringnumber() + pcre_get_stringtable_entries() + + pcre_free_substring() and pcre_free_substring_list() are also provided, + to free the memory used for extracted strings. + + The function pcre_maketables() is used to build a set of character + tables in the current locale for passing to pcre_compile(), + pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is + provided for specialist use. Most commonly, no special tables are + passed, in which case internal tables that are generated when PCRE is + built are used. + + The function pcre_fullinfo() is used to find out information about a + compiled pattern; pcre_info() is an obsolete version that returns only + some of the available information, but is retained for backwards com- + patibility. The function pcre_version() returns a pointer to a string + containing the version of PCRE and its date of release. + + The function pcre_refcount() maintains a reference count in a data + block containing a compiled pattern. This is provided for the benefit + of object-oriented applications. + + The global variables pcre_malloc and pcre_free initially contain the + entry points of the standard malloc() and free() functions, respec- + tively. PCRE calls the memory management functions via these variables, + so a calling program can replace them if it wishes to intercept the + calls. This should be done before calling any PCRE functions. + + The global variables pcre_stack_malloc and pcre_stack_free are also + indirections to memory management functions. These special functions + are used only when PCRE is compiled to use the heap for remembering + data, instead of recursive function calls, when running the pcre_exec() + function. See the pcrebuild documentation for details of how to do + this. It is a non-standard way of building PCRE, for use in environ- + ments that have limited stacks. Because of the greater use of memory + management, it runs more slowly. Separate functions are provided so + that special-purpose external code can be used for this case. When + used, these functions are always called in a stack-like manner (last + obtained, first freed), and always for memory blocks of the same size. + There is a discussion about PCRE's stack usage in the pcrestack docu- + mentation. + + The global variable pcre_callout initially contains NULL. It can be set + by the caller to a "callout" function, which PCRE will then call at + specified points during a matching operation. Details are given in the + pcrecallout documentation. + + +NEWLINES + + PCRE supports five different conventions for indicating line breaks in + strings: a single CR (carriage return) character, a single LF (line- + feed) character, the two-character sequence CRLF, any of the three pre- + ceding, or any Unicode newline sequence. The Unicode newline sequences + are the three just mentioned, plus the single characters VT (vertical + tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line + separator, U+2028), and PS (paragraph separator, U+2029). + + Each of the first three conventions is used by at least one operating + system as its standard newline sequence. When PCRE is built, a default + can be specified. The default default is LF, which is the Unix stan- + dard. When PCRE is run, the default can be overridden, either when a + pattern is compiled, or when it is matched. + + At compile time, the newline convention can be specified by the options + argument of pcre_compile(), or it can be specified by special text at + the start of the pattern itself; this overrides any other settings. See + the pcrepattern page for details of the special character sequences. + + In the PCRE documentation the word "newline" is used to mean "the char- + acter or pair of characters that indicate a line break". The choice of + newline convention affects the handling of the dot, circumflex, and + dollar metacharacters, the handling of #-comments in /x mode, and, when + CRLF is a recognized line ending sequence, the match position advance- + ment for a non-anchored pattern. There is more detail about this in the + section on pcre_exec() options below. + + The choice of newline convention does not affect the interpretation of + the \n or \r escape sequences, nor does it affect what \R matches, + which is controlled in a similar way, but by separate options. + + +MULTITHREADING + + The PCRE functions can be used in multi-threading applications, with + the proviso that the memory management functions pointed to by + pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the + callout function pointed to by pcre_callout, are shared by all threads. + + The compiled form of a regular expression is not altered during match- + ing, so the same compiled pattern can safely be used by several threads + at once. + + +SAVING PRECOMPILED PATTERNS FOR LATER USE + + The compiled form of a regular expression can be saved and re-used at a + later time, possibly by a different program, and even on a host other + than the one on which it was compiled. Details are given in the + pcreprecompile documentation. However, compiling a regular expression + with one version of PCRE for use with a different version is not guar- + anteed to work and may cause crashes. + + +CHECKING BUILD-TIME OPTIONS + + int pcre_config(int what, void *where); + + The function pcre_config() makes it possible for a PCRE client to dis- + cover which optional features have been compiled into the PCRE library. + The pcrebuild documentation has more details about these optional fea- + tures. + + The first argument for pcre_config() is an integer, specifying which + information is required; the second argument is a pointer to a variable + into which the information is placed. The following information is + available: + + PCRE_CONFIG_UTF8 + + The output is an integer that is set to one if UTF-8 support is avail- + able; otherwise it is set to zero. + + PCRE_CONFIG_UNICODE_PROPERTIES + + The output is an integer that is set to one if support for Unicode + character properties is available; otherwise it is set to zero. + + PCRE_CONFIG_NEWLINE + + The output is an integer whose value specifies the default character + sequence that is recognized as meaning "newline". The four values that + are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, + and -1 for ANY. The default should normally be the standard sequence + for your operating system. + + PCRE_CONFIG_BSR + + The output is an integer whose value indicates what character sequences + the \R escape sequence matches by default. A value of 0 means that \R + matches any Unicode line ending sequence; a value of 1 means that \R + matches only CR, LF, or CRLF. The default can be overridden when a pat- + tern is compiled or matched. + + PCRE_CONFIG_LINK_SIZE + + The output is an integer that contains the number of bytes used for + internal linkage in compiled regular expressions. The value is 2, 3, or + 4. Larger values allow larger regular expressions to be compiled, at + the expense of slower matching. The default value of 2 is sufficient + for all but the most massive patterns, since it allows the compiled + pattern to be up to 64K in size. + + PCRE_CONFIG_POSIX_MALLOC_THRESHOLD + + The output is an integer that contains the threshold above which the + POSIX interface uses malloc() for output vectors. Further details are + given in the pcreposix documentation. + + PCRE_CONFIG_MATCH_LIMIT + + The output is an integer that gives the default limit for the number of + internal matching function calls in a pcre_exec() execution. Further + details are given with pcre_exec() below. + + PCRE_CONFIG_MATCH_LIMIT_RECURSION + + The output is an integer that gives the default limit for the depth of + recursion when calling the internal matching function in a pcre_exec() + execution. Further details are given with pcre_exec() below. + + PCRE_CONFIG_STACKRECURSE + + The output is an integer that is set to one if internal recursion when + running pcre_exec() is implemented by recursive function calls that use + the stack to remember their state. This is the usual way that PCRE is + compiled. The output is zero if PCRE was compiled to use blocks of data + on the heap instead of recursive function calls. In this case, + pcre_stack_malloc and pcre_stack_free are called to manage memory + blocks on the heap, thus avoiding the use of the stack. + + +COMPILING A PATTERN + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre *pcre_compile2(const char *pattern, int options, + int *errorcodeptr, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + Either of the functions pcre_compile() or pcre_compile2() can be called + to compile a pattern into an internal form. The only difference between + the two interfaces is that pcre_compile2() has an additional argument, + errorcodeptr, via which a numerical error code can be returned. + + The pattern is a C string terminated by a binary zero, and is passed in + the pattern argument. A pointer to a single block of memory that is + obtained via pcre_malloc is returned. This contains the compiled code + and related data. The pcre type is defined for the returned block; this + is a typedef for a structure whose contents are not externally defined. + It is up to the caller to free the memory (via pcre_free) when it is no + longer required. + + Although the compiled code of a PCRE regex is relocatable, that is, it + does not depend on memory location, the complete pcre data block is not + fully relocatable, because it may contain a copy of the tableptr argu- + ment, which is an address (see below). + + The options argument contains various bit settings that affect the com- + pilation. It should be zero if no options are required. The available + options are described below. Some of them, in particular, those that + are compatible with Perl, can also be set and unset from within the + pattern (see the detailed description in the pcrepattern documenta- + tion). For these options, the contents of the options argument speci- + fies their initial settings at the start of compilation and execution. + The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time + of matching as well as at compile time. + + If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, + if compilation of a pattern fails, pcre_compile() returns NULL, and + sets the variable pointed to by errptr to point to a textual error mes- + sage. This is a static string that is part of the library. You must not + try to free it. The offset from the start of the pattern to the charac- + ter where the error was discovered is placed in the variable pointed to + by erroffset, which must not be NULL. If it is, an immediate error is + given. + + If pcre_compile2() is used instead of pcre_compile(), and the error- + codeptr argument is not NULL, a non-zero error code number is returned + via this argument in the event of an error. This is in addition to the + textual error message. Error codes and messages are listed below. + + If the final argument, tableptr, is NULL, PCRE uses a default set of + character tables that are built when PCRE is compiled, using the + default C locale. Otherwise, tableptr must be an address that is the + result of a call to pcre_maketables(). This value is stored with the + compiled pattern, and used again by pcre_exec(), unless another table + pointer is passed to it. For more discussion, see the section on locale + support below. + + This code fragment shows a typical straightforward call to pcre_com- + pile(): + + pcre *re; + const char *error; + int erroffset; + re = pcre_compile( + "^A.*Z", /* the pattern */ + 0, /* default options */ + &error, /* for error message */ + &erroffset, /* for error offset */ + NULL); /* use default character tables */ + + The following names for option bits are defined in the pcre.h header + file: + + PCRE_ANCHORED + + If this bit is set, the pattern is forced to be "anchored", that is, it + is constrained to match only at the first matching point in the string + that is being searched (the "subject string"). This effect can also be + achieved by appropriate constructs in the pattern itself, which is the + only way to do it in Perl. + + PCRE_AUTO_CALLOUT + + If this bit is set, pcre_compile() automatically inserts callout items, + all with number 255, before each pattern item. For discussion of the + callout facility, see the pcrecallout documentation. + + PCRE_BSR_ANYCRLF + PCRE_BSR_UNICODE + + These options (which are mutually exclusive) control what the \R escape + sequence matches. The choice is either to match only CR, LF, or CRLF, + or to match any Unicode newline sequence. The default is specified when + PCRE is built. It can be overridden from within the pattern, or by set- + ting an option when a compiled pattern is matched. + + PCRE_CASELESS + + If this bit is set, letters in the pattern match both upper and lower + case letters. It is equivalent to Perl's /i option, and it can be + changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE + always understands the concept of case for characters whose values are + less than 128, so caseless matching is always possible. For characters + with higher values, the concept of case is supported if PCRE is com- + piled with Unicode property support, but not otherwise. If you want to + use caseless matching for characters 128 and above, you must ensure + that PCRE is compiled with Unicode property support as well as with + UTF-8 support. + + PCRE_DOLLAR_ENDONLY + + If this bit is set, a dollar metacharacter in the pattern matches only + at the end of the subject string. Without this option, a dollar also + matches immediately before a newline at the end of the string (but not + before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored + if PCRE_MULTILINE is set. There is no equivalent to this option in + Perl, and no way to set it within a pattern. + + PCRE_DOTALL + + If this bit is set, a dot metacharater in the pattern matches all char- + acters, including those that indicate newline. Without it, a dot does + not match when the current position is at a newline. This option is + equivalent to Perl's /s option, and it can be changed within a pattern + by a (?s) option setting. A negative class such as [^a] always matches + newline characters, independent of the setting of this option. + + PCRE_DUPNAMES + + If this bit is set, names used to identify capturing subpatterns need + not be unique. This can be helpful for certain types of pattern when it + is known that only one instance of the named subpattern can ever be + matched. There are more details of named subpatterns below; see also + the pcrepattern documentation. + + PCRE_EXTENDED + + If this bit is set, whitespace data characters in the pattern are + totally ignored except when escaped or inside a character class. White- + space does not include the VT character (code 11). In addition, charac- + ters between an unescaped # outside a character class and the next new- + line, inclusive, are also ignored. This is equivalent to Perl's /x + option, and it can be changed within a pattern by a (?x) option set- + ting. + + This option makes it possible to include comments inside complicated + patterns. Note, however, that this applies only to data characters. + Whitespace characters may never appear within special character + sequences in a pattern, for example within the sequence (?( which + introduces a conditional subpattern. + + PCRE_EXTRA + + This option was invented in order to turn on additional functionality + of PCRE that is incompatible with Perl, but it is currently of very + little use. When set, any backslash in a pattern that is followed by a + letter that has no special meaning causes an error, thus reserving + these combinations for future expansion. By default, as in Perl, a + backslash followed by a letter with no special meaning is treated as a + literal. (Perl can, however, be persuaded to give a warning for this.) + There are at present no other features controlled by this option. It + can also be set by a (?X) option setting within a pattern. + + PCRE_FIRSTLINE + + If this option is set, an unanchored pattern is required to match + before or at the first newline in the subject string, though the + matched text may continue over the newline. + + PCRE_JAVASCRIPT_COMPAT + + If this option is set, PCRE's behaviour is changed in some ways so that + it is compatible with JavaScript rather than Perl. The changes are as + follows: + + (1) A lone closing square bracket in a pattern causes a compile-time + error, because this is illegal in JavaScript (by default it is treated + as a data character). Thus, the pattern AB]CD becomes illegal when this + option is set. + + (2) At run time, a back reference to an unset subpattern group matches + an empty string (by default this causes the current matching alterna- + tive to fail). A pattern such as (\1)(a) succeeds when this option is + set (assuming it can find an "a" in the subject), whereas it fails by + default, for Perl compatibility. + + PCRE_MULTILINE + + By default, PCRE treats the subject string as consisting of a single + line of characters (even if it actually contains newlines). The "start + of line" metacharacter (^) matches only at the start of the string, + while the "end of line" metacharacter ($) matches only at the end of + the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY + is set). This is the same as Perl. + + When PCRE_MULTILINE it is set, the "start of line" and "end of line" + constructs match immediately following or immediately before internal + newlines in the subject string, respectively, as well as at the very + start and end. This is equivalent to Perl's /m option, and it can be + changed within a pattern by a (?m) option setting. If there are no new- + lines in a subject string, or no occurrences of ^ or $ in a pattern, + setting PCRE_MULTILINE has no effect. + + PCRE_NEWLINE_CR + PCRE_NEWLINE_LF + PCRE_NEWLINE_CRLF + PCRE_NEWLINE_ANYCRLF + PCRE_NEWLINE_ANY + + These options override the default newline definition that was chosen + when PCRE was built. Setting the first or the second specifies that a + newline is indicated by a single character (CR or LF, respectively). + Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the + two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies + that any of the three preceding sequences should be recognized. Setting + PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be + recognized. The Unicode newline sequences are the three just mentioned, + plus the single characters VT (vertical tab, U+000B), FF (formfeed, + U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS + (paragraph separator, U+2029). The last two are recognized only in + UTF-8 mode. + + The newline setting in the options word uses three bits that are + treated as a number, giving eight possibilities. Currently only six are + used (default plus the five values above). This means that if you set + more than one newline option, the combination may or may not be sensi- + ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to + PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and + cause an error. + + The only time that a line break is specially recognized when compiling + a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a + character class is encountered. This indicates a comment that lasts + until after the next line break sequence. In other circumstances, line + break sequences are treated as literal data, except that in + PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters + and are therefore ignored. + + The newline option that is set at compile time becomes the default that + is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. + + PCRE_NO_AUTO_CAPTURE + + If this option is set, it disables the use of numbered capturing paren- + theses in the pattern. Any opening parenthesis that is not followed by + ? behaves as if it were followed by ?: but named parentheses can still + be used for capturing (and they acquire numbers in the usual way). + There is no equivalent of this option in Perl. + + PCRE_UNGREEDY + + This option inverts the "greediness" of the quantifiers so that they + are not greedy by default, but become greedy if followed by "?". It is + not compatible with Perl. It can also be set by a (?U) option setting + within the pattern. + + PCRE_UTF8 + + This option causes PCRE to regard both the pattern and the subject as + strings of UTF-8 characters instead of single-byte character strings. + However, it is available only when PCRE is built to include UTF-8 sup- + port. If not, the use of this option provokes an error. Details of how + this option changes the behaviour of PCRE are given in the section on + UTF-8 support in the main pcre page. + + PCRE_NO_UTF8_CHECK + + When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is + automatically checked. There is a discussion about the validity of + UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of + bytes is found, pcre_compile() returns an error. If you already know + that your pattern is valid, and you want to skip this check for perfor- + mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is + set, the effect of passing an invalid UTF-8 string as a pattern is + undefined. It may cause your program to crash. Note that this option + can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the + UTF-8 validity checking of subject strings. + + +COMPILATION ERROR CODES + + The following table lists the error codes than may be returned by + pcre_compile2(), along with the error messages that may be returned by + both compiling functions. As PCRE has developed, some error codes have + fallen out of use. To avoid confusion, they have not been re-used. + + 0 no error + 1 \ at end of pattern + 2 \c at end of pattern + 3 unrecognized character follows \ + 4 numbers out of order in {} quantifier + 5 number too big in {} quantifier + 6 missing terminating ] for character class + 7 invalid escape sequence in character class + 8 range out of order in character class + 9 nothing to repeat + 10 [this code is not in use] + 11 internal error: unexpected repeat + 12 unrecognized character after (? or (?- + 13 POSIX named classes are supported only within a class + 14 missing ) + 15 reference to non-existent subpattern + 16 erroffset passed as NULL + 17 unknown option bit(s) set + 18 missing ) after comment + 19 [this code is not in use] + 20 regular expression is too large + 21 failed to get memory + 22 unmatched parentheses + 23 internal error: code overflow + 24 unrecognized character after (?< + 25 lookbehind assertion is not fixed length + 26 malformed number or name after (?( + 27 conditional group contains more than two branches + 28 assertion expected after (?( + 29 (?R or (?[+-]digits must be followed by ) + 30 unknown POSIX class name + 31 POSIX collating elements are not supported + 32 this version of PCRE is not compiled with PCRE_UTF8 support + 33 [this code is not in use] + 34 character value in \x{...} sequence is too large + 35 invalid condition (?(0) + 36 \C not allowed in lookbehind assertion + 37 PCRE does not support \L, \l, \N, \U, or \u + 38 number after (?C is > 255 + 39 closing ) for (?C expected + 40 recursive call could loop indefinitely + 41 unrecognized character after (?P + 42 syntax error in subpattern name (missing terminator) + 43 two named subpatterns have the same name + 44 invalid UTF-8 string + 45 support for \P, \p, and \X has not been compiled + 46 malformed \P or \p sequence + 47 unknown property name after \P or \p + 48 subpattern name is too long (maximum 32 characters) + 49 too many named subpatterns (maximum 10000) + 50 [this code is not in use] + 51 octal value is greater than \377 (not in UTF-8 mode) + 52 internal error: overran compiling workspace + 53 internal error: previously-checked referenced subpattern not + found + 54 DEFINE group contains more than one branch + 55 repeating a DEFINE group is not allowed + 56 inconsistent NEWLINE options + 57 \g is not followed by a braced, angle-bracketed, or quoted + name/number or by a plain number + 58 a numbered reference must not be zero + 59 (*VERB) with an argument is not supported + 60 (*VERB) not recognized + 61 number is too big + 62 subpattern name expected + 63 digit expected after (?+ + 64 ] is an invalid data character in JavaScript compatibility mode + + The numbers 32 and 10000 in errors 48 and 49 are defaults; different + values may be used if the limits were changed when PCRE was built. + + +STUDYING A PATTERN + + pcre_extra *pcre_study(const pcre *code, int options + const char **errptr); + + If a compiled pattern is going to be used several times, it is worth + spending more time analyzing it in order to speed up the time taken for + matching. The function pcre_study() takes a pointer to a compiled pat- + tern as its first argument. If studying the pattern produces additional + information that will help speed up matching, pcre_study() returns a + pointer to a pcre_extra block, in which the study_data field points to + the results of the study. + + The returned value from pcre_study() can be passed directly to + pcre_exec(). However, a pcre_extra block also contains other fields + that can be set by the caller before the block is passed; these are + described below in the section on matching a pattern. + + If studying the pattern does not produce any additional information + pcre_study() returns NULL. In that circumstance, if the calling program + wants to pass any of the other fields to pcre_exec(), it must set up + its own pcre_extra block. + + The second argument of pcre_study() contains option bits. At present, + no options are defined, and this argument should always be zero. + + The third argument for pcre_study() is a pointer for an error message. + If studying succeeds (even if no data is returned), the variable it + points to is set to NULL. Otherwise it is set to point to a textual + error message. This is a static string that is part of the library. You + must not try to free it. You should test the error pointer for NULL + after calling pcre_study(), to be sure that it has run successfully. + + This is a typical call to pcre_study(): + + pcre_extra *pe; + pe = pcre_study( + re, /* result of pcre_compile() */ + 0, /* no options exist */ + &error); /* set to NULL or points to a message */ + + At present, studying a pattern is useful only for non-anchored patterns + that do not have a single fixed starting character. A bitmap of possi- + ble starting bytes is created. + + +LOCALE SUPPORT + + PCRE handles caseless matching, and determines whether characters are + letters, digits, or whatever, by reference to a set of tables, indexed + by character value. When running in UTF-8 mode, this applies only to + characters with codes less than 128. Higher-valued codes never match + escapes such as \w or \d, but can be tested with \p if PCRE is built + with Unicode character property support. The use of locales with Uni- + code is discouraged. If you are handling characters with codes greater + than 128, you should either use UTF-8 and Unicode, or use locales, but + not try to mix the two. + + PCRE contains an internal set of tables that are used when the final + argument of pcre_compile() is NULL. These are sufficient for many + applications. Normally, the internal tables recognize only ASCII char- + acters. However, when PCRE is built, it is possible to cause the inter- + nal tables to be rebuilt in the default "C" locale of the local system, + which may cause them to be different. + + The internal tables can always be overridden by tables supplied by the + application that calls PCRE. These may be created in a different locale + from the default. As more and more applications change to using Uni- + code, the need for this locale support is expected to die away. + + External tables are built by calling the pcre_maketables() function, + which has no arguments, in the relevant locale. The result can then be + passed to pcre_compile() or pcre_exec() as often as necessary. For + example, to build and use tables that are appropriate for the French + locale (where accented characters with values greater than 128 are + treated as letters), the following code could be used: + + setlocale(LC_CTYPE, "fr_FR"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); + + The locale name "fr_FR" is used on Linux and other Unix-like systems; + if you are using Windows, the name for the French locale is "french". + + When pcre_maketables() runs, the tables are built in memory that is + obtained via pcre_malloc. It is the caller's responsibility to ensure + that the memory containing the tables remains available for as long as + it is needed. + + The pointer that is passed to pcre_compile() is saved with the compiled + pattern, and the same tables are used via this pointer by pcre_study() + and normally also by pcre_exec(). Thus, by default, for any single pat- + tern, compilation, studying and matching all happen in the same locale, + but different patterns can be compiled in different locales. + + It is possible to pass a table pointer or NULL (indicating the use of + the internal tables) to pcre_exec(). Although not intended for this + purpose, this facility could be used to match a pattern in a different + locale from the one in which it was compiled. Passing table pointers at + run time is discussed below in the section on matching a pattern. + + +INFORMATION ABOUT A PATTERN + + int pcre_fullinfo(const pcre *code, const pcre_extra *extra, + int what, void *where); + + The pcre_fullinfo() function returns information about a compiled pat- + tern. It replaces the obsolete pcre_info() function, which is neverthe- + less retained for backwards compability (and is documented below). + + The first argument for pcre_fullinfo() is a pointer to the compiled + pattern. The second argument is the result of pcre_study(), or NULL if + the pattern was not studied. The third argument specifies which piece + of information is required, and the fourth argument is a pointer to a + variable to receive the data. The yield of the function is zero for + success, or one of the following negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + the argument where was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + PCRE_ERROR_BADOPTION the value of what was invalid + + The "magic number" is placed at the start of each compiled pattern as + an simple check against passing an arbitrary memory pointer. Here is a + typical call of pcre_fullinfo(), to obtain the length of the compiled + pattern: + + int rc; + size_t length; + rc = pcre_fullinfo( + re, /* result of pcre_compile() */ + pe, /* result of pcre_study(), or NULL */ + PCRE_INFO_SIZE, /* what is required */ + &length); /* where to put the data */ + + The possible values for the third argument are defined in pcre.h, and + are as follows: + + PCRE_INFO_BACKREFMAX + + Return the number of the highest back reference in the pattern. The + fourth argument should point to an int variable. Zero is returned if + there are no back references. + + PCRE_INFO_CAPTURECOUNT + + Return the number of capturing subpatterns in the pattern. The fourth + argument should point to an int variable. + + PCRE_INFO_DEFAULT_TABLES + + Return a pointer to the internal default character tables within PCRE. + The fourth argument should point to an unsigned char * variable. This + information call is provided for internal use by the pcre_study() func- + tion. External callers can cause PCRE to use its internal tables by + passing a NULL table pointer. + + PCRE_INFO_FIRSTBYTE + + Return information about the first byte of any matched string, for a + non-anchored pattern. The fourth argument should point to an int vari- + able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name + is still recognized for backwards compatibility.) + + If there is a fixed first byte, for example, from a pattern such as + (cat|cow|coyote), its value is returned. Otherwise, if either + + (a) the pattern was compiled with the PCRE_MULTILINE option, and every + branch starts with "^", or + + (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not + set (if it were set, the pattern would be anchored), + + -1 is returned, indicating that the pattern matches only at the start + of a subject string or after any newline within the string. Otherwise + -2 is returned. For anchored patterns, -2 is returned. + + PCRE_INFO_FIRSTTABLE + + If the pattern was studied, and this resulted in the construction of a + 256-bit table indicating a fixed set of bytes for the first byte in any + matching string, a pointer to the table is returned. Otherwise NULL is + returned. The fourth argument should point to an unsigned char * vari- + able. + + PCRE_INFO_HASCRORLF + + Return 1 if the pattern contains any explicit matches for CR or LF + characters, otherwise 0. The fourth argument should point to an int + variable. An explicit match is either a literal CR or LF character, or + \r or \n. + + PCRE_INFO_JCHANGED + + Return 1 if the (?J) or (?-J) option setting is used in the pattern, + otherwise 0. The fourth argument should point to an int variable. (?J) + and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. + + PCRE_INFO_LASTLITERAL + + Return the value of the rightmost literal byte that must exist in any + matched string, other than at its start, if such a byte has been + recorded. The fourth argument should point to an int variable. If there + is no such byte, -1 is returned. For anchored patterns, a last literal + byte is recorded only if it follows something of variable length. For + example, for the pattern /^a\d+z\d+/ the returned value is "z", but for + /^a\dz\d/ the returned value is -1. + + PCRE_INFO_NAMECOUNT + PCRE_INFO_NAMEENTRYSIZE + PCRE_INFO_NAMETABLE + + PCRE supports the use of named as well as numbered capturing parenthe- + ses. The names are just an additional way of identifying the parenthe- + ses, which still acquire numbers. Several convenience functions such as + pcre_get_named_substring() are provided for extracting captured sub- + strings by name. It is also possible to extract the data directly, by + first converting the name to a number in order to access the correct + pointers in the output vector (described with pcre_exec() below). To do + the conversion, you need to use the name-to-number map, which is + described by these three values. + + The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT + gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size + of each entry; both of these return an int value. The entry size + depends on the length of the longest name. PCRE_INFO_NAMETABLE returns + a pointer to the first entry of the table (a pointer to char). The + first two bytes of each entry are the number of the capturing parenthe- + sis, most significant byte first. The rest of the entry is the corre- + sponding name, zero terminated. The names are in alphabetical order. + When PCRE_DUPNAMES is set, duplicate names are in order of their paren- + theses numbers. For example, consider the following pattern (assume + PCRE_EXTENDED is set, so white space - including newlines - is + ignored): + + (? (?(\d\d)?\d\d) - + (?\d\d) - (?\d\d) ) + + There are four named subpatterns, so the table has four entries, and + each entry in the table is eight bytes long. The table is as follows, + with non-printing bytes shows in hexadecimal, and undefined bytes shown + as ??: + + 00 01 d a t e 00 ?? + 00 05 d a y 00 ?? ?? + 00 04 m o n t h 00 + 00 02 y e a r 00 ?? + + When writing code to extract data from named subpatterns using the + name-to-number map, remember that the length of the entries is likely + to be different for each compiled pattern. + + PCRE_INFO_OKPARTIAL + + Return 1 if the pattern can be used for partial matching, otherwise 0. + The fourth argument should point to an int variable. The pcrepartial + documentation lists the restrictions that apply to patterns when par- + tial matching is used. + + PCRE_INFO_OPTIONS + + Return a copy of the options with which the pattern was compiled. The + fourth argument should point to an unsigned long int variable. These + option bits are those specified in the call to pcre_compile(), modified + by any top-level option settings at the start of the pattern itself. In + other words, they are the options that will be in force when matching + starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with + the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, + and PCRE_EXTENDED. + + A pattern is automatically anchored by PCRE if all of its top-level + alternatives begin with one of the following: + + ^ unless PCRE_MULTILINE is set + \A always + \G always + .* if PCRE_DOTALL is set and there are no back + references to the subpattern in which .* appears + + For such patterns, the PCRE_ANCHORED bit is set in the options returned + by pcre_fullinfo(). + + PCRE_INFO_SIZE + + Return the size of the compiled pattern, that is, the value that was + passed as the argument to pcre_malloc() when PCRE was getting memory in + which to place the compiled data. The fourth argument should point to a + size_t variable. + + PCRE_INFO_STUDYSIZE + + Return the size of the data block pointed to by the study_data field in + a pcre_extra block. That is, it is the value that was passed to + pcre_malloc() when PCRE was getting memory into which to place the data + created by pcre_study(). The fourth argument should point to a size_t + variable. + + +OBSOLETE INFO FUNCTION + + int pcre_info(const pcre *code, int *optptr, int *firstcharptr); + + The pcre_info() function is now obsolete because its interface is too + restrictive to return all the available data about a compiled pattern. + New programs should use pcre_fullinfo() instead. The yield of + pcre_info() is the number of capturing subpatterns, or one of the fol- + lowing negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + + If the optptr argument is not NULL, a copy of the options with which + the pattern was compiled is placed in the integer it points to (see + PCRE_INFO_OPTIONS above). + + If the pattern is not anchored and the firstcharptr argument is not + NULL, it is used to pass back information about the first character of + any matched string (see PCRE_INFO_FIRSTBYTE above). + + +REFERENCE COUNTS + + int pcre_refcount(pcre *code, int adjust); + + The pcre_refcount() function is used to maintain a reference count in + the data block that contains a compiled pattern. It is provided for the + benefit of applications that operate in an object-oriented manner, + where different parts of the application may be using the same compiled + pattern, but you want to free the block when they are all done. + + When a pattern is compiled, the reference count field is initialized to + zero. It is changed only by calling this function, whose action is to + add the adjust value (which may be positive or negative) to it. The + yield of the function is the new value. However, the value of the count + is constrained to lie between 0 and 65535, inclusive. If the new value + is outside these limits, it is forced to the appropriate limit value. + + Except when it is zero, the reference count is not correctly preserved + if a pattern is compiled on one host and then transferred to a host + whose byte-order is different. (This seems a highly unlikely scenario.) + + +MATCHING A PATTERN: THE TRADITIONAL FUNCTION + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + The function pcre_exec() is called to match a subject string against a + compiled pattern, which is passed in the code argument. If the pattern + has been studied, the result of the study should be passed in the extra + argument. This function is the main matching facility of the library, + and it operates in a Perl-like manner. For specialist use there is also + an alternative matching function, which is described below in the sec- + tion about the pcre_dfa_exec() function. + + In most applications, the pattern will have been compiled (and option- + ally studied) in the same process that calls pcre_exec(). However, it + is possible to save compiled patterns and study data, and then use them + later in different processes, possibly even on different hosts. For a + discussion about this, see the pcreprecompile documentation. + + Here is an example of a simple call to pcre_exec(): + + int rc; + int ovector[30]; + rc = pcre_exec( + re, /* result of pcre_compile() */ + NULL, /* we didn't study the pattern */ + "some string", /* the subject string */ + 11, /* the length of the subject string */ + 0, /* start at offset 0 in the subject */ + 0, /* default options */ + ovector, /* vector of integers for substring information */ + 30); /* number of elements (NOT size in bytes) */ + + Extra data for pcre_exec() + + If the extra argument is not NULL, it must point to a pcre_extra data + block. The pcre_study() function returns such a block (when it doesn't + return NULL), but you can also create one for yourself, and pass addi- + tional information in it. The pcre_extra block contains the following + fields (not necessarily in this order): + + unsigned long int flags; + void *study_data; + unsigned long int match_limit; + unsigned long int match_limit_recursion; + void *callout_data; + const unsigned char *tables; + + The flags field is a bitmap that specifies which of the other fields + are set. The flag bits are: + + PCRE_EXTRA_STUDY_DATA + PCRE_EXTRA_MATCH_LIMIT + PCRE_EXTRA_MATCH_LIMIT_RECURSION + PCRE_EXTRA_CALLOUT_DATA + PCRE_EXTRA_TABLES + + Other flag bits should be set to zero. The study_data field is set in + the pcre_extra block that is returned by pcre_study(), together with + the appropriate flag bit. You should not set this yourself, but you may + add to the block by setting the other fields and their corresponding + flag bits. + + The match_limit field provides a means of preventing PCRE from using up + a vast amount of resources when running patterns that are not going to + match, but which have a very large number of possibilities in their + search trees. The classic example is the use of nested unlimited + repeats. + + Internally, PCRE uses a function called match() which it calls repeat- + edly (sometimes recursively). The limit set by match_limit is imposed + on the number of times this function is called during a match, which + has the effect of limiting the amount of backtracking that can take + place. For patterns that are not anchored, the count restarts from zero + for each position in the subject string. + + The default value for the limit can be set when PCRE is built; the + default default is 10 million, which handles all but the most extreme + cases. You can override the default by suppling pcre_exec() with a + pcre_extra block in which match_limit is set, and + PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is + exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. + + The match_limit_recursion field is similar to match_limit, but instead + of limiting the total number of times that match() is called, it limits + the depth of recursion. The recursion depth is a smaller number than + the total number of calls, because not all calls to match() are recur- + sive. This limit is of use only if it is set smaller than match_limit. + + Limiting the recursion depth limits the amount of stack that can be + used, or, when PCRE has been compiled to use memory on the heap instead + of the stack, the amount of heap memory that can be used. + + The default value for match_limit_recursion can be set when PCRE is + built; the default default is the same value as the default for + match_limit. You can override the default by suppling pcre_exec() with + a pcre_extra block in which match_limit_recursion is set, and + PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the + limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. + + The pcre_callout field is used in conjunction with the "callout" fea- + ture, which is described in the pcrecallout documentation. + + The tables field is used to pass a character tables pointer to + pcre_exec(); this overrides the value that is stored with the compiled + pattern. A non-NULL value is stored with the compiled pattern only if + custom tables were supplied to pcre_compile() via its tableptr argu- + ment. If NULL is passed to pcre_exec() using this mechanism, it forces + PCRE's internal tables to be used. This facility is helpful when re- + using patterns that have been saved after compiling with an external + set of tables, because the external tables might be at a different + address when pcre_exec() is called. See the pcreprecompile documenta- + tion for a discussion of saving compiled patterns for later use. + + Option bits for pcre_exec() + + The unused bits of the options argument for pcre_exec() must be zero. + The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, + PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and + PCRE_PARTIAL. + + PCRE_ANCHORED + + The PCRE_ANCHORED option limits pcre_exec() to matching at the first + matching position. If a pattern was compiled with PCRE_ANCHORED, or + turned out to be anchored by virtue of its contents, it cannot be made + unachored at matching time. + + PCRE_BSR_ANYCRLF + PCRE_BSR_UNICODE + + These options (which are mutually exclusive) control what the \R escape + sequence matches. The choice is either to match only CR, LF, or CRLF, + or to match any Unicode newline sequence. These options override the + choice that was made or defaulted when the pattern was compiled. + + PCRE_NEWLINE_CR + PCRE_NEWLINE_LF + PCRE_NEWLINE_CRLF + PCRE_NEWLINE_ANYCRLF + PCRE_NEWLINE_ANY + + These options override the newline definition that was chosen or + defaulted when the pattern was compiled. For details, see the descrip- + tion of pcre_compile() above. During matching, the newline choice + affects the behaviour of the dot, circumflex, and dollar metacharac- + ters. It may also alter the way the match position is advanced after a + match failure for an unanchored pattern. + + When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is + set, and a match attempt for an unanchored pattern fails when the cur- + rent position is at a CRLF sequence, and the pattern contains no + explicit matches for CR or LF characters, the match position is + advanced by two characters instead of one, in other words, to after the + CRLF. + + The above rule is a compromise that makes the most common cases work as + expected. For example, if the pattern is .+A (and the PCRE_DOTALL + option is not set), it does not match the string "\r\nA" because, after + failing at the start, it skips both the CR and the LF before retrying. + However, the pattern [\r\n]A does match that string, because it con- + tains an explicit CR or LF reference, and so advances only by one char- + acter after the first failure. + + An explicit match for CR of LF is either a literal appearance of one of + those characters, or one of the \r or \n escape sequences. Implicit + matches such as [^X] do not count, nor does \s (which includes CR and + LF in the characters that it matches). + + Notwithstanding the above, anomalous effects may still occur when CRLF + is a valid newline sequence and explicit \r or \n escapes appear in the + pattern. + + PCRE_NOTBOL + + This option specifies that first character of the subject string is not + the beginning of a line, so the circumflex metacharacter should not + match before it. Setting this without PCRE_MULTILINE (at compile time) + causes circumflex never to match. This option affects only the behav- + iour of the circumflex metacharacter. It does not affect \A. + + PCRE_NOTEOL + + This option specifies that the end of the subject string is not the end + of a line, so the dollar metacharacter should not match it nor (except + in multiline mode) a newline immediately before it. Setting this with- + out PCRE_MULTILINE (at compile time) causes dollar never to match. This + option affects only the behaviour of the dollar metacharacter. It does + not affect \Z or \z. + + PCRE_NOTEMPTY + + An empty string is not considered to be a valid match if this option is + set. If there are alternatives in the pattern, they are tried. If all + the alternatives match the empty string, the entire match fails. For + example, if the pattern + + a?b? + + is applied to a string not beginning with "a" or "b", it matches the + empty string at the start of the subject. With PCRE_NOTEMPTY set, this + match is not valid, so PCRE searches further into the string for occur- + rences of "a" or "b". + + Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe- + cial case of a pattern match of the empty string within its split() + function, and when using the /g modifier. It is possible to emulate + Perl's behaviour after matching a null string by first trying the match + again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then + if that fails by advancing the starting offset (see below) and trying + an ordinary match again. There is some code that demonstrates how to do + this in the pcredemo.c sample program. + + PCRE_NO_UTF8_CHECK + + When PCRE_UTF8 is set at compile time, the validity of the subject as a + UTF-8 string is automatically checked when pcre_exec() is subsequently + called. The value of startoffset is also checked to ensure that it + points to the start of a UTF-8 character. There is a discussion about + the validity of UTF-8 strings in the section on UTF-8 support in the + main pcre page. If an invalid UTF-8 sequence of bytes is found, + pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con- + tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned. + + If you already know that your subject is valid, and you want to skip + these checks for performance reasons, you can set the + PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to + do this for the second and subsequent calls to pcre_exec() if you are + making repeated calls to find all the matches in a single subject + string. However, you should be sure that the value of startoffset + points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is + set, the effect of passing an invalid UTF-8 string as a subject, or a + value of startoffset that does not point to the start of a UTF-8 char- + acter, is undefined. Your program may crash. + + PCRE_PARTIAL + + This option turns on the partial matching feature. If the subject + string fails to match the pattern, but at some point during the match- + ing process the end of the subject was reached (that is, the subject + partially matches the pattern and the failure to match occurred only + because there were not enough subject characters), pcre_exec() returns + PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is + used, there are restrictions on what may appear in the pattern. These + are discussed in the pcrepartial documentation. + + The string to be matched by pcre_exec() + + The subject string is passed to pcre_exec() as a pointer in subject, a + length (in bytes) in length, and a starting byte offset in startoffset. + In UTF-8 mode, the byte offset must point to the start of a UTF-8 char- + acter. Unlike the pattern string, the subject may contain binary zero + bytes. When the starting offset is zero, the search for a match starts + at the beginning of the subject, and this is by far the most common + case. + + A non-zero starting offset is useful when searching for another match + in the same subject by calling pcre_exec() again after a previous suc- + cess. Setting startoffset differs from just passing over a shortened + string and setting PCRE_NOTBOL in the case of a pattern that begins + with any kind of lookbehind. For example, consider the pattern + + \Biss\B + + which finds occurrences of "iss" in the middle of words. (\B matches + only if the current position in the subject is not a word boundary.) + When applied to the string "Mississipi" the first call to pcre_exec() + finds the first occurrence. If pcre_exec() is called again with just + the remainder of the subject, namely "issipi", it does not match, + because \B is always false at the start of the subject, which is deemed + to be a word boundary. However, if pcre_exec() is passed the entire + string again, but with startoffset set to 4, it finds the second occur- + rence of "iss" because it is able to look behind the starting point to + discover that it is preceded by a letter. + + If a non-zero starting offset is passed when the pattern is anchored, + one attempt to match at the given offset is made. This can only succeed + if the pattern does not require the match to be at the start of the + subject. + + How pcre_exec() returns captured substrings + + In general, a pattern matches a certain portion of the subject, and in + addition, further substrings from the subject may be picked out by + parts of the pattern. Following the usage in Jeffrey Friedl's book, + this is called "capturing" in what follows, and the phrase "capturing + subpattern" is used for a fragment of a pattern that picks out a sub- + string. PCRE supports several other kinds of parenthesized subpattern + that do not cause substrings to be captured. + + Captured substrings are returned to the caller via a vector of integers + whose address is passed in ovector. The number of elements in the vec- + tor is passed in ovecsize, which must be a non-negative number. Note: + this argument is NOT the size of ovector in bytes. + + The first two-thirds of the vector is used to pass back captured sub- + strings, each substring using a pair of integers. The remaining third + of the vector is used as workspace by pcre_exec() while matching cap- + turing subpatterns, and is not available for passing back information. + The number passed in ovecsize should always be a multiple of three. If + it is not, it is rounded down. + + When a match is successful, information about captured substrings is + returned in pairs of integers, starting at the beginning of ovector, + and continuing up to two-thirds of its length at the most. The first + element of each pair is set to the byte offset of the first character + in a substring, and the second is set to the byte offset of the first + character after the end of a substring. Note: these values are always + byte offsets, even in UTF-8 mode. They are not character counts. + + The first pair of integers, ovector[0] and ovector[1], identify the + portion of the subject string matched by the entire pattern. The next + pair is used for the first capturing subpattern, and so on. The value + returned by pcre_exec() is one more than the highest numbered pair that + has been set. For example, if two substrings have been captured, the + returned value is 3. If there are no capturing subpatterns, the return + value from a successful match is 1, indicating that just the first pair + of offsets has been set. + + If a capturing subpattern is matched repeatedly, it is the last portion + of the string that it matched that is returned. + + If the vector is too small to hold all the captured substring offsets, + it is used as far as possible (up to two-thirds of its length), and the + function returns a value of zero. If the substring offsets are not of + interest, pcre_exec() may be called with ovector passed as NULL and + ovecsize as zero. However, if the pattern contains back references and + the ovector is not big enough to remember the related substrings, PCRE + has to get additional memory for use during matching. Thus it is usu- + ally advisable to supply an ovector. + + The pcre_info() function can be used to find out how many capturing + subpatterns there are in a compiled pattern. The smallest size for + ovector that will allow for n captured substrings, in addition to the + offsets of the substring matched by the whole pattern, is (n+1)*3. + + It is possible for capturing subpattern number n+1 to match some part + of the subject when subpattern n has not been used at all. For example, + if the string "abc" is matched against the pattern (a|(z))(bc) the + return from the function is 4, and subpatterns 1 and 3 are matched, but + 2 is not. When this happens, both values in the offset pairs corre- + sponding to unused subpatterns are set to -1. + + Offset values that correspond to unused subpatterns at the end of the + expression are also set to -1. For example, if the string "abc" is + matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not + matched. The return from the function is 2, because the highest used + capturing subpattern number is 1. However, you can refer to the offsets + for the second and third capturing subpatterns if you wish (assuming + the vector is large enough, of course). + + Some convenience functions are provided for extracting the captured + substrings as separate strings. These are described below. + + Error return values from pcre_exec() + + If pcre_exec() fails, it returns a negative number. The following are + defined in the header file: + + PCRE_ERROR_NOMATCH (-1) + + The subject string did not match the pattern. + + PCRE_ERROR_NULL (-2) + + Either code or subject was passed as NULL, or ovector was NULL and + ovecsize was not zero. + + PCRE_ERROR_BADOPTION (-3) + + An unrecognized bit was set in the options argument. + + PCRE_ERROR_BADMAGIC (-4) + + PCRE stores a 4-byte "magic number" at the start of the compiled code, + to catch the case when it is passed a junk pointer and to detect when a + pattern that was compiled in an environment of one endianness is run in + an environment with the other endianness. This is the error that PCRE + gives when the magic number is not present. + + PCRE_ERROR_UNKNOWN_OPCODE (-5) + + While running the pattern match, an unknown item was encountered in the + compiled pattern. This error could be caused by a bug in PCRE or by + overwriting of the compiled pattern. + + PCRE_ERROR_NOMEMORY (-6) + + If a pattern contains back references, but the ovector that is passed + to pcre_exec() is not big enough to remember the referenced substrings, + PCRE gets a block of memory at the start of matching to use for this + purpose. If the call via pcre_malloc() fails, this error is given. The + memory is automatically freed at the end of matching. + + PCRE_ERROR_NOSUBSTRING (-7) + + This error is used by the pcre_copy_substring(), pcre_get_substring(), + and pcre_get_substring_list() functions (see below). It is never + returned by pcre_exec(). + + PCRE_ERROR_MATCHLIMIT (-8) + + The backtracking limit, as specified by the match_limit field in a + pcre_extra structure (or defaulted) was reached. See the description + above. + + PCRE_ERROR_CALLOUT (-9) + + This error is never generated by pcre_exec() itself. It is provided for + use by callout functions that want to yield a distinctive error code. + See the pcrecallout documentation for details. + + PCRE_ERROR_BADUTF8 (-10) + + A string that contains an invalid UTF-8 byte sequence was passed as a + subject. + + PCRE_ERROR_BADUTF8_OFFSET (-11) + + The UTF-8 byte sequence that was passed as a subject was valid, but the + value of startoffset did not point to the beginning of a UTF-8 charac- + ter. + + PCRE_ERROR_PARTIAL (-12) + + The subject string did not match, but it did match partially. See the + pcrepartial documentation for details of partial matching. + + PCRE_ERROR_BADPARTIAL (-13) + + The PCRE_PARTIAL option was used with a compiled pattern containing + items that are not supported for partial matching. See the pcrepartial + documentation for details of partial matching. + + PCRE_ERROR_INTERNAL (-14) + + An unexpected internal error has occurred. This error could be caused + by a bug in PCRE or by overwriting of the compiled pattern. + + PCRE_ERROR_BADCOUNT (-15) + + This error is given if the value of the ovecsize argument is negative. + + PCRE_ERROR_RECURSIONLIMIT (-21) + + The internal recursion limit, as specified by the match_limit_recursion + field in a pcre_extra structure (or defaulted) was reached. See the + description above. + + PCRE_ERROR_BADNEWLINE (-23) + + An invalid combination of PCRE_NEWLINE_xxx options was given. + + Error numbers -16 to -20 and -22 are not used by pcre_exec(). + + +EXTRACTING CAPTURED SUBSTRINGS BY NUMBER + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + Captured substrings can be accessed directly by using the offsets + returned by pcre_exec() in ovector. For convenience, the functions + pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- + string_list() are provided for extracting captured substrings as new, + separate, zero-terminated strings. These functions identify substrings + by number. The next section describes functions for extracting named + substrings. + + A substring that contains a binary zero is correctly extracted and has + a further zero added on the end, but the result is not, of course, a C + string. However, you can process such a string by referring to the + length that is returned by pcre_copy_substring() and pcre_get_sub- + string(). Unfortunately, the interface to pcre_get_substring_list() is + not adequate for handling strings containing binary zeros, because the + end of the final string is not independently indicated. + + The first three arguments are the same for all three of these func- + tions: subject is the subject string that has just been successfully + matched, ovector is a pointer to the vector of integer offsets that was + passed to pcre_exec(), and stringcount is the number of substrings that + were captured by the match, including the substring that matched the + entire regular expression. This is the value returned by pcre_exec() if + it is greater than zero. If pcre_exec() returned zero, indicating that + it ran out of space in ovector, the value passed as stringcount should + be the number of elements in the vector divided by three. + + The functions pcre_copy_substring() and pcre_get_substring() extract a + single substring, whose number is given as stringnumber. A value of + zero extracts the substring that matched the entire pattern, whereas + higher values extract the captured substrings. For pcre_copy_sub- + string(), the string is placed in buffer, whose length is given by + buffersize, while for pcre_get_substring() a new block of memory is + obtained via pcre_malloc, and its address is returned via stringptr. + The yield of the function is the length of the string, not including + the terminating zero, or one of these error codes: + + PCRE_ERROR_NOMEMORY (-6) + + The buffer was too small for pcre_copy_substring(), or the attempt to + get memory failed for pcre_get_substring(). + + PCRE_ERROR_NOSUBSTRING (-7) + + There is no substring whose number is stringnumber. + + The pcre_get_substring_list() function extracts all available sub- + strings and builds a list of pointers to them. All this is done in a + single block of memory that is obtained via pcre_malloc. The address of + the memory block is returned via listptr, which is also the start of + the list of string pointers. The end of the list is marked by a NULL + pointer. The yield of the function is zero if all went well, or the + error code + + PCRE_ERROR_NOMEMORY (-6) + + if the attempt to get the memory block failed. + + When any of these functions encounter a substring that is unset, which + can happen when capturing subpattern number n+1 matches some part of + the subject, but subpattern n has not been used at all, they return an + empty string. This can be distinguished from a genuine zero-length sub- + string by inspecting the appropriate offset in ovector, which is nega- + tive for unset substrings. + + The two convenience functions pcre_free_substring() and pcre_free_sub- + string_list() can be used to free the memory returned by a previous + call of pcre_get_substring() or pcre_get_substring_list(), respec- + tively. They do nothing more than call the function pointed to by + pcre_free, which of course could be called directly from a C program. + However, PCRE is used in some situations where it is linked via a spe- + cial interface to another programming language that cannot use + pcre_free directly; it is for these cases that the functions are pro- + vided. + + +EXTRACTING CAPTURED SUBSTRINGS BY NAME + + int pcre_get_stringnumber(const pcre *code, + const char *name); + + int pcre_copy_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + char *buffer, int buffersize); + + int pcre_get_named_substring(const pcre *code, + const char *subject, int *ovector, + int stringcount, const char *stringname, + const char **stringptr); + + To extract a substring by name, you first have to find associated num- + ber. For example, for this pattern + + (a+)b(?\d+)... + + the number of the subpattern called "xxx" is 2. If the name is known to + be unique (PCRE_DUPNAMES was not set), you can find the number from the + name by calling pcre_get_stringnumber(). The first argument is the com- + piled pattern, and the second is the name. The yield of the function is + the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no + subpattern of that name. + + Given the number, you can extract the substring directly, or use one of + the functions described in the previous section. For convenience, there + are also two functions that do the whole job. + + Most of the arguments of pcre_copy_named_substring() and + pcre_get_named_substring() are the same as those for the similarly + named functions that extract by number. As these are described in the + previous section, they are not re-described here. There are just two + differences: + + First, instead of a substring number, a substring name is given. Sec- + ond, there is an extra argument, given at the start, which is a pointer + to the compiled pattern. This is needed in order to gain access to the + name-to-number translation table. + + These functions call pcre_get_stringnumber(), and if it succeeds, they + then call pcre_copy_substring() or pcre_get_substring(), as appropri- + ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the + behaviour may not be what you want (see the next section). + + +DUPLICATE SUBPATTERN NAMES + + int pcre_get_stringtable_entries(const pcre *code, + const char *name, char **first, char **last); + + When a pattern is compiled with the PCRE_DUPNAMES option, names for + subpatterns are not required to be unique. Normally, patterns with + duplicate names are such that in any one match, only one of the named + subpatterns participates. An example is shown in the pcrepattern docu- + mentation. + + When duplicates are present, pcre_copy_named_substring() and + pcre_get_named_substring() return the first substring corresponding to + the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING + (-7) is returned; no data is returned. The pcre_get_stringnumber() + function returns one of the numbers that are associated with the name, + but it is not defined which it is. + + If you want to get full details of all captured substrings for a given + name, you must use the pcre_get_stringtable_entries() function. The + first argument is the compiled pattern, and the second is the name. The + third and fourth are pointers to variables which are updated by the + function. After it has run, they point to the first and last entries in + the name-to-number table for the given name. The function itself + returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if + there are none. The format of the table is described above in the sec- + tion entitled Information about a pattern. Given all the relevant + entries for the name, you can extract each of their numbers, and hence + the captured data, if any. + + +FINDING ALL POSSIBLE MATCHES + + The traditional matching function uses a similar algorithm to Perl, + which stops when it finds the first match, starting at a given point in + the subject. If you want to find all possible matches, or the longest + possible match, consider using the alternative matching function (see + below) instead. If you cannot use the alternative function, but still + need to find all possible matches, you can kludge it up by making use + of the callout facility, which is described in the pcrecallout documen- + tation. + + What you have to do is to insert a callout right at the end of the pat- + tern. When your callout function is called, extract and save the cur- + rent matched substring. Then return 1, which forces pcre_exec() to + backtrack and try other alternatives. Ultimately, when it runs out of + matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. + + +MATCHING A PATTERN: THE ALTERNATIVE FUNCTION + + int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize, + int *workspace, int wscount); + + The function pcre_dfa_exec() is called to match a subject string + against a compiled pattern, using a matching algorithm that scans the + subject string just once, and does not backtrack. This has different + characteristics to the normal algorithm, and is not compatible with + Perl. Some of the features of PCRE patterns are not supported. Never- + theless, there are times when this kind of matching can be useful. For + a discussion of the two matching algorithms, see the pcrematching docu- + mentation. + + The arguments for the pcre_dfa_exec() function are the same as for + pcre_exec(), plus two extras. The ovector argument is used in a differ- + ent way, and this is described below. The other common arguments are + used in the same way as for pcre_exec(), so their description is not + repeated here. + + The two additional arguments provide workspace for the function. The + workspace vector should contain at least 20 elements. It is used for + keeping track of multiple paths through the pattern tree. More + workspace will be needed for patterns and subjects where there are a + lot of potential matches. + + Here is an example of a simple call to pcre_dfa_exec(): + + int rc; + int ovector[10]; + int wspace[20]; + rc = pcre_dfa_exec( + re, /* result of pcre_compile() */ + NULL, /* we didn't study the pattern */ + "some string", /* the subject string */ + 11, /* the length of the subject string */ + 0, /* start at offset 0 in the subject */ + 0, /* default options */ + ovector, /* vector of integers for substring information */ + 10, /* number of elements (NOT size in bytes) */ + wspace, /* working space vector */ + 20); /* number of elements (NOT size in bytes) */ + + Option bits for pcre_dfa_exec() + + The unused bits of the options argument for pcre_dfa_exec() must be + zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- + LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK, + PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last + three of these are the same as for pcre_exec(), so their description is + not repeated here. + + PCRE_PARTIAL + + This has the same general effect as it does for pcre_exec(), but the + details are slightly different. When PCRE_PARTIAL is set for + pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into + PCRE_ERROR_PARTIAL if the end of the subject is reached, there have + been no complete matches, but there is still at least one matching pos- + sibility. The portion of the string that provided the partial match is + set as the first matching string. + + PCRE_DFA_SHORTEST + + Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to + stop as soon as it has found one match. Because of the way the alterna- + tive algorithm works, this is necessarily the shortest possible match + at the first possible matching point in the subject string. + + PCRE_DFA_RESTART + + When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and + returns a partial match, it is possible to call it again, with addi- + tional subject characters, and have it continue with the same match. + The PCRE_DFA_RESTART option requests this action; when it is set, the + workspace and wscount options must reference the same vector as before + because data about the match so far is left in them after a partial + match. There is more discussion of this facility in the pcrepartial + documentation. + + Successful returns from pcre_dfa_exec() + + When pcre_dfa_exec() succeeds, it may have matched more than one sub- + string in the subject. Note, however, that all the matches from one run + of the function start at the same point in the subject. The shorter + matches are all initial substrings of the longer matches. For example, + if the pattern + + <.*> + + is matched against the string + + This is no more + + the three matched strings are + + + + + + On success, the yield of the function is a number greater than zero, + which is the number of matched substrings. The substrings themselves + are returned in ovector. Each string uses two elements; the first is + the offset to the start, and the second is the offset to the end. In + fact, all the strings have the same start offset. (Space could have + been saved by giving this only once, but it was decided to retain some + compatibility with the way pcre_exec() returns data, even though the + meaning of the strings is different.) + + The strings are returned in reverse order of length; that is, the long- + est matching string is given first. If there were too many matches to + fit into ovector, the yield of the function is zero, and the vector is + filled with the longest matches. + + Error returns from pcre_dfa_exec() + + The pcre_dfa_exec() function returns a negative number when it fails. + Many of the errors are the same as for pcre_exec(), and these are + described above. There are in addition the following errors that are + specific to pcre_dfa_exec(): + + PCRE_ERROR_DFA_UITEM (-16) + + This return is given if pcre_dfa_exec() encounters an item in the pat- + tern that it does not support, for instance, the use of \C or a back + reference. + + PCRE_ERROR_DFA_UCOND (-17) + + This return is given if pcre_dfa_exec() encounters a condition item + that uses a back reference for the condition, or a test for recursion + in a specific group. These are not supported. + + PCRE_ERROR_DFA_UMLIMIT (-18) + + This return is given if pcre_dfa_exec() is called with an extra block + that contains a setting of the match_limit field. This is not supported + (it is meaningless). + + PCRE_ERROR_DFA_WSSIZE (-19) + + This return is given if pcre_dfa_exec() runs out of space in the + workspace vector. + + PCRE_ERROR_DFA_RECURSE (-20) + + When a recursive subpattern is processed, the matching function calls + itself recursively, using private vectors for ovector and workspace. + This error is given if the output vector is not large enough. This + should be extremely rare, as a vector of size 1000 is used. + + +SEE ALSO + + pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar- + tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3). + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 24 August 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCRECALLOUT(3) PCRECALLOUT(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE CALLOUTS + + int (*pcre_callout)(pcre_callout_block *); + + PCRE provides a feature called "callout", which is a means of temporar- + ily passing control to the caller of PCRE in the middle of pattern + matching. The caller of PCRE provides an external function by putting + its entry point in the global variable pcre_callout. By default, this + variable contains NULL, which disables all calling out. + + Within a regular expression, (?C) indicates the points at which the + external function is to be called. Different callout points can be + identified by putting a number less than 256 after the letter C. The + default value is zero. For example, this pattern has two callout + points: + + (?C1)abc(?C2)def + + If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is + called, PCRE automatically inserts callouts, all with number 255, + before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is + used with the pattern + + A(\d{2}|--) + + it is processed as if it were + + (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) + + Notice that there is a callout before and after each parenthesis and + alternation bar. Automatic callouts can be used for tracking the + progress of pattern matching. The pcretest command has an option that + sets automatic callouts; when it is used, the output indicates how the + pattern is matched. This is useful information when you are trying to + optimize the performance of a particular pattern. + + +MISSING CALLOUTS + + You should be aware that, because of optimizations in the way PCRE + matches patterns, callouts sometimes do not happen. For example, if the + pattern is + + ab(?C4)cd + + PCRE knows that any matching string must contain the letter "d". If the + subject string is "abyz", the lack of "d" means that matching doesn't + ever start, and the callout is never reached. However, with "abyd", + though the result is still no match, the callout is obeyed. + + +THE CALLOUT INTERFACE + + During matching, when PCRE reaches a callout point, the external func- + tion defined by pcre_callout is called (if it is set). This applies to + both the pcre_exec() and the pcre_dfa_exec() matching functions. The + only argument to the callout function is a pointer to a pcre_callout + block. This structure contains the following fields: + + int version; + int callout_number; + int *offset_vector; + const char *subject; + int subject_length; + int start_match; + int current_position; + int capture_top; + int capture_last; + void *callout_data; + int pattern_position; + int next_item_length; + + The version field is an integer containing the version number of the + block format. The initial version was 0; the current version is 1. The + version number will change again in future if additional fields are + added, but the intention is never to remove any of the existing fields. + + The callout_number field contains the number of the callout, as com- + piled into the pattern (that is, the number after ?C for manual call- + outs, and 255 for automatically generated callouts). + + The offset_vector field is a pointer to the vector of offsets that was + passed by the caller to pcre_exec() or pcre_dfa_exec(). When + pcre_exec() is used, the contents can be inspected in order to extract + substrings that have been matched so far, in the same way as for + extracting substrings after a match has completed. For pcre_dfa_exec() + this field is not useful. + + The subject and subject_length fields contain copies of the values that + were passed to pcre_exec(). + + The start_match field normally contains the offset within the subject + at which the current match attempt started. However, if the escape + sequence \K has been encountered, this value is changed to reflect the + modified starting point. If the pattern is not anchored, the callout + function may be called several times from the same point in the pattern + for different starting points in the subject. + + The current_position field contains the offset within the subject of + the current match pointer. + + When the pcre_exec() function is used, the capture_top field contains + one more than the number of the highest numbered captured substring so + far. If no substrings have been captured, the value of capture_top is + one. This is always the case when pcre_dfa_exec() is used, because it + does not support captured substrings. + + The capture_last field contains the number of the most recently cap- + tured substring. If no substrings have been captured, its value is -1. + This is always the case when pcre_dfa_exec() is used. + + The callout_data field contains a value that is passed to pcre_exec() + or pcre_dfa_exec() specifically so that it can be passed back in call- + outs. It is passed in the pcre_callout field of the pcre_extra data + structure. If no such data was passed, the value of callout_data in a + pcre_callout block is NULL. There is a description of the pcre_extra + structure in the pcreapi documentation. + + The pattern_position field is present from version 1 of the pcre_call- + out structure. It contains the offset to the next item to be matched in + the pattern string. + + The next_item_length field is present from version 1 of the pcre_call- + out structure. It contains the length of the next item to be matched in + the pattern string. When the callout immediately precedes an alterna- + tion bar, a closing parenthesis, or the end of the pattern, the length + is zero. When the callout precedes an opening parenthesis, the length + is that of the entire subpattern. + + The pattern_position and next_item_length fields are intended to help + in distinguishing between different automatic callouts, which all have + the same callout number. However, they are set for all callouts. + + +RETURN VALUES + + The external callout function returns an integer to PCRE. If the value + is zero, matching proceeds as normal. If the value is greater than + zero, matching fails at the current point, but the testing of other + matching possibilities goes ahead, just as if a lookahead assertion had + failed. If the value is less than zero, the match is abandoned, and + pcre_exec() (or pcre_dfa_exec()) returns the negative value. + + Negative values should normally be chosen from the set of + PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- + dard "no match" failure. The error number PCRE_ERROR_CALLOUT is + reserved for use by callout functions; it will never be used by PCRE + itself. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 29 May 2007 + Copyright (c) 1997-2007 University of Cambridge. +------------------------------------------------------------------------------ + + +PCRECOMPAT(3) PCRECOMPAT(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +DIFFERENCES BETWEEN PCRE AND PERL + + This document describes the differences in the ways that PCRE and Perl + handle regular expressions. The differences described here are mainly + with respect to Perl 5.8, though PCRE versions 7.0 and later contain + some features that are expected to be in the forthcoming Perl 5.10. + + 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details + of what it does have are given in the section on UTF-8 support in the + main pcre page. + + 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl + permits them, but they do not mean what you might think. For example, + (?!a){3} does not assert that the next three characters are not "a". It + just asserts that the next character is not "a" three times. + + 3. Capturing subpatterns that occur inside negative lookahead asser- + tions are counted, but their entries in the offsets vector are never + set. Perl sets its numerical variables from any such patterns that are + matched before the assertion fails to match something (thereby succeed- + ing), but only if the negative lookahead assertion contains just one + branch. + + 4. Though binary zero characters are supported in the subject string, + they are not allowed in a pattern string because it is passed as a nor- + mal C string, terminated by zero. The escape sequence \0 can be used in + the pattern to represent a binary zero. + + 5. The following Perl escape sequences are not supported: \l, \u, \L, + \U, and \N. In fact these are implemented by Perl's general string-han- + dling and are not part of its pattern matching engine. If any of these + are encountered by PCRE, an error is generated. + + 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE + is built with Unicode character property support. The properties that + can be tested with \p and \P are limited to the general category prop- + erties such as Lu and Nd, script names such as Greek or Han, and the + derived properties Any and L&. + + 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- + ters in between are treated as literals. This is slightly different + from Perl in that $ and @ are also handled as literals inside the + quotes. In Perl, they cause variable interpolation (but of course PCRE + does not have variables). Note the following examples: + + Pattern PCRE matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the + contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz + + The \Q...\E sequence is recognized both inside and outside character + classes. + + 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) + constructions. However, there is support for recursive patterns. This + is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE + "callout" feature allows an external function to be called during pat- + tern matching. See the pcrecallout documentation for details. + + 9. Subpatterns that are called recursively or as "subroutines" are + always treated as atomic groups in PCRE. This is like Python, but + unlike Perl. + + 10. There are some differences that are concerned with the settings of + captured strings when part of a pattern is repeated. For example, + matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 + unset, but in PCRE it is set to "b". + + 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), + (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in + the forms without an argument. PCRE does not support (*MARK). If + (*ACCEPT) is within capturing parentheses, PCRE does not set that cap- + ture group; this is different to Perl. + + 12. PCRE provides some extensions to the Perl regular expression facil- + ities. Perl 5.10 will include new features that are not in earlier + versions, some of which (such as named parentheses) have been in PCRE + for some time. This list is with respect to Perl 5.10: + + (a) Although lookbehind assertions must match fixed length strings, + each alternative branch of a lookbehind assertion can match a different + length of string. Perl requires them all to have the same length. + + (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ + meta-character matches only at the very end of the string. + + (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- + cial meaning is faulted. Otherwise, like Perl, the backslash is quietly + ignored. (Perl can be made to issue a warning.) + + (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- + fiers is inverted, that is, by default they are not greedy, but if fol- + lowed by a question mark they are. + + (e) PCRE_ANCHORED can be used at matching time to force a pattern to be + tried only at the first matching position in the subject string. + + (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP- + TURE options for pcre_exec() have no Perl equivalents. + + (g) The \R escape sequence can be restricted to match only CR, LF, or + CRLF by the PCRE_BSR_ANYCRLF option. + + (h) The callout facility is PCRE-specific. + + (i) The partial matching facility is PCRE-specific. + + (j) Patterns compiled by PCRE can be saved and re-used at a later time, + even on different hosts that have the other endianness. + + (k) The alternative matching function (pcre_dfa_exec()) matches in a + different way and is not Perl-compatible. + + (l) PCRE recognizes some special sequences such as (*CR) at the start + of a pattern that set overall options that cannot be changed within the + pattern. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 11 September 2007 + Copyright (c) 1997-2007 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREPATTERN(3) PCREPATTERN(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE REGULAR EXPRESSION DETAILS + + The syntax and semantics of the regular expressions that are supported + by PCRE are described in detail below. There is a quick-reference syn- + tax summary in the pcresyntax page. PCRE tries to match Perl syntax and + semantics as closely as it can. PCRE also supports some alternative + regular expression syntax (which does not conflict with the Perl syn- + tax) in order to provide some compatibility with regular expressions in + Python, .NET, and Oniguruma. + + Perl's regular expressions are described in its own documentation, and + regular expressions in general are covered in a number of books, some + of which have copious examples. Jeffrey Friedl's "Mastering Regular + Expressions", published by O'Reilly, covers regular expressions in + great detail. This description of PCRE's regular expressions is + intended as reference material. + + The original operation of PCRE was on strings of one-byte characters. + However, there is now also support for UTF-8 character strings. To use + this, you must build PCRE to include UTF-8 support, and then call + pcre_compile() with the PCRE_UTF8 option. How this affects pattern + matching is mentioned in several places below. There is also a summary + of UTF-8 features in the section on UTF-8 support in the main pcre + page. + + The remainder of this document discusses the patterns that are sup- + ported by PCRE when its main matching function, pcre_exec(), is used. + From release 6.0, PCRE offers a second matching function, + pcre_dfa_exec(), which matches using a different algorithm that is not + Perl-compatible. Some of the features discussed below are not available + when pcre_dfa_exec() is used. The advantages and disadvantages of the + alternative function, and how it differs from the normal function, are + discussed in the pcrematching page. + + +NEWLINE CONVENTIONS + + PCRE supports five different conventions for indicating line breaks in + strings: a single CR (carriage return) character, a single LF (line- + feed) character, the two-character sequence CRLF, any of the three pre- + ceding, or any Unicode newline sequence. The pcreapi page has further + discussion about newlines, and shows how to set the newline convention + in the options arguments for the compiling and matching functions. + + It is also possible to specify a newline convention by starting a pat- + tern string with one of the following five sequences: + + (*CR) carriage return + (*LF) linefeed + (*CRLF) carriage return, followed by linefeed + (*ANYCRLF) any of the three above + (*ANY) all Unicode newline sequences + + These override the default and the options given to pcre_compile(). For + example, on a Unix system where LF is the default newline sequence, the + pattern + + (*CR)a.b + + changes the convention to CR. That pattern matches "a\nb" because LF is + no longer a newline. Note that these special settings, which are not + Perl-compatible, are recognized only at the very start of a pattern, + and that they must be in upper case. If more than one of them is + present, the last one is used. + + The newline convention does not affect what the \R escape sequence + matches. By default, this is any Unicode newline sequence, for Perl + compatibility. However, this can be changed; see the description of \R + in the section entitled "Newline sequences" below. A change of \R set- + ting can be combined with a change of newline convention. + + +CHARACTERS AND METACHARACTERS + + A regular expression is a pattern that is matched against a subject + string from left to right. Most characters stand for themselves in a + pattern, and match the corresponding characters in the subject. As a + trivial example, the pattern + + The quick brown fox + + matches a portion of a subject string that is identical to itself. When + caseless matching is specified (the PCRE_CASELESS option), letters are + matched independently of case. In UTF-8 mode, PCRE always understands + the concept of case for characters whose values are less than 128, so + caseless matching is always possible. For characters with higher val- + ues, the concept of case is supported if PCRE is compiled with Unicode + property support, but not otherwise. If you want to use caseless + matching for characters 128 and above, you must ensure that PCRE is + compiled with Unicode property support as well as with UTF-8 support. + + The power of regular expressions comes from the ability to include + alternatives and repetitions in the pattern. These are encoded in the + pattern by the use of metacharacters, which do not stand for themselves + but instead are interpreted in some special way. + + There are two different sets of metacharacters: those that are recog- + nized anywhere in the pattern except within square brackets, and those + that are recognized within square brackets. Outside square brackets, + the metacharacters are as follows: + + \ general escape character with several uses + ^ assert start of string (or line, in multiline mode) + $ assert end of string (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + also "possessive quantifier" + { start min/max quantifier + + Part of a pattern that is in square brackets is called a "character + class". In a character class the only metacharacters are: + + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + [ POSIX character class (only if followed by POSIX + syntax) + ] terminates the character class + + The following sections describe the use of each of the metacharacters. + + +BACKSLASH + + The backslash character has several uses. Firstly, if it is followed by + a non-alphanumeric character, it takes away any special meaning that + character may have. This use of backslash as an escape character + applies both inside and outside character classes. + + For example, if you want to match a * character, you write \* in the + pattern. This escaping action applies whether or not the following + character would otherwise be interpreted as a metacharacter, so it is + always safe to precede a non-alphanumeric with backslash to specify + that it stands for itself. In particular, if you want to match a back- + slash, you write \\. + + If a pattern is compiled with the PCRE_EXTENDED option, whitespace in + the pattern (other than in a character class) and characters between a + # outside a character class and the next newline are ignored. An escap- + ing backslash can be used to include a whitespace or # character as + part of the pattern. + + If you want to remove the special meaning from a sequence of charac- + ters, you can do so by putting them between \Q and \E. This is differ- + ent from Perl in that $ and @ are handled as literals in \Q...\E + sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- + tion. Note the following examples: + + Pattern PCRE matches Perl matches + + \Qabc$xyz\E abc$xyz abc followed by the + contents of $xyz + \Qabc\$xyz\E abc\$xyz abc\$xyz + \Qabc\E\$\Qxyz\E abc$xyz abc$xyz + + The \Q...\E sequence is recognized both inside and outside character + classes. + + Non-printing characters + + A second use of backslash provides a way of encoding non-printing char- + acters in patterns in a visible manner. There is no restriction on the + appearance of non-printing characters, apart from the binary zero that + terminates a pattern, but when a pattern is being prepared by text + editing, it is usually easier to use one of the following escape + sequences than the binary character it represents: + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n linefeed (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \ddd character with octal code ddd, or backreference + \xhh character with hex code hh + \x{hhh..} character with hex code hhh.. + + The precise effect of \cx is as follows: if x is a lower case letter, + it is converted to upper case. Then bit 6 of the character (hex 40) is + inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; + becomes hex 7B. + + After \x, from zero to two hexadecimal digits are read (letters can be + in upper or lower case). Any number of hexadecimal digits may appear + between \x{ and }, but the value of the character code must be less + than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, + the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger + than the largest Unicode code point, which is 10FFFF. + + If characters other than hexadecimal digits appear between \x{ and }, + or if there is no terminating }, this form of escape is not recognized. + Instead, the initial \x will be interpreted as a basic hexadecimal + escape, with no following digits, giving a character whose value is + zero. + + Characters whose value is less than 256 can be defined by either of the + two syntaxes for \x. There is no difference in the way they are han- + dled. For example, \xdc is exactly the same as \x{dc}. + + After \0 up to two further octal digits are read. If there are fewer + than two digits, just those that are present are used. Thus the + sequence \0\x\07 specifies two binary zeros followed by a BEL character + (code value 7). Make sure you supply two digits after the initial zero + if the pattern character that follows is itself an octal digit. + + The handling of a backslash followed by a digit other than 0 is compli- + cated. Outside a character class, PCRE reads it and any following dig- + its as a decimal number. If the number is less than 10, or if there + have been at least that many previous capturing left parentheses in the + expression, the entire sequence is taken as a back reference. A + description of how this works is given later, following the discussion + of parenthesized subpatterns. + + Inside a character class, or if the decimal number is greater than 9 + and there have not been that many capturing subpatterns, PCRE re-reads + up to three octal digits following the backslash, and uses them to gen- + erate a data character. Any subsequent digits stand for themselves. In + non-UTF-8 mode, the value of a character specified in octal must be + less than \400. In UTF-8 mode, values up to \777 are permitted. For + example: + + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 might be a back reference, otherwise the + character with octal code 113 + \377 might be a back reference, otherwise + the byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + + Note that octal values of 100 or greater must not be introduced by a + leading zero, because no more than three octal digits are ever read. + + All the sequences that define a single character value can be used both + inside and outside character classes. In addition, inside a character + class, the sequence \b is interpreted as the backspace character (hex + 08), and the sequences \R and \X are interpreted as the characters "R" + and "X", respectively. Outside a character class, these sequences have + different meanings (see below). + + Absolute and relative back references + + The sequence \g followed by an unsigned or a negative number, option- + ally enclosed in braces, is an absolute or relative back reference. A + named back reference can be coded as \g{name}. Back references are dis- + cussed later, following the discussion of parenthesized subpatterns. + + Absolute and relative subroutine calls + + For compatibility with Oniguruma, the non-Perl syntax \g followed by a + name or a number enclosed either in angle brackets or single quotes, is + an alternative syntax for referencing a subpattern as a "subroutine". + Details are discussed later. Note that \g{...} (Perl syntax) and + \g<...> (Oniguruma syntax) are not synonymous. The former is a back + reference; the latter is a subroutine call. + + Generic character types + + Another use of backslash is for specifying generic character types. The + following are always recognized: + + \d any decimal digit + \D any character that is not a decimal digit + \h any horizontal whitespace character + \H any character that is not a horizontal whitespace character + \s any whitespace character + \S any character that is not a whitespace character + \v any vertical whitespace character + \V any character that is not a vertical whitespace character + \w any "word" character + \W any "non-word" character + + Each pair of escape sequences partitions the complete set of characters + into two disjoint sets. Any given character matches one, and only one, + of each pair. + + These character type sequences can appear both inside and outside char- + acter classes. They each match one character of the appropriate type. + If the current matching point is at the end of the subject string, all + of them fail, since there is no character to match. + + For compatibility with Perl, \s does not match the VT character (code + 11). This makes it different from the the POSIX "space" class. The \s + characters are HT (9), LF (10), FF (12), CR (13), and space (32). If + "use locale;" is included in a Perl script, \s may match the VT charac- + ter. In PCRE, it never does. + + In UTF-8 mode, characters with values greater than 128 never match \d, + \s, or \w, and always match \D, \S, and \W. This is true even when Uni- + code character property support is available. These sequences retain + their original meanings from before UTF-8 support was available, mainly + for efficiency reasons. + + The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to + the other sequences, these do match certain high-valued codepoints in + UTF-8 mode. The horizontal space characters are: + + U+0009 Horizontal tab + U+0020 Space + U+00A0 Non-break space + U+1680 Ogham space mark + U+180E Mongolian vowel separator + U+2000 En quad + U+2001 Em quad + U+2002 En space + U+2003 Em space + U+2004 Three-per-em space + U+2005 Four-per-em space + U+2006 Six-per-em space + U+2007 Figure space + U+2008 Punctuation space + U+2009 Thin space + U+200A Hair space + U+202F Narrow no-break space + U+205F Medium mathematical space + U+3000 Ideographic space + + The vertical space characters are: + + U+000A Linefeed + U+000B Vertical tab + U+000C Formfeed + U+000D Carriage return + U+0085 Next line + U+2028 Line separator + U+2029 Paragraph separator + + A "word" character is an underscore or any character less than 256 that + is a letter or digit. The definition of letters and digits is con- + trolled by PCRE's low-valued character tables, and may vary if locale- + specific matching is taking place (see "Locale support" in the pcreapi + page). For example, in a French locale such as "fr_FR" in Unix-like + systems, or "french" in Windows, some character codes greater than 128 + are used for accented letters, and these are matched by \w. The use of + locales with Unicode is discouraged. + + Newline sequences + + Outside a character class, by default, the escape sequence \R matches + any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 + mode \R is equivalent to the following: + + (?>\r\n|\n|\x0b|\f|\r|\x85) + + This is an example of an "atomic group", details of which are given + below. This particular group matches either the two-character sequence + CR followed by LF, or one of the single characters LF (linefeed, + U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage + return, U+000D), or NEL (next line, U+0085). The two-character sequence + is treated as a single unit that cannot be split. + + In UTF-8 mode, two additional characters whose codepoints are greater + than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- + rator, U+2029). Unicode character property support is not needed for + these characters to be recognized. + + It is possible to restrict \R to match only CR, LF, or CRLF (instead of + the complete set of Unicode line endings) by setting the option + PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. + (BSR is an abbrevation for "backslash R".) This can be made the default + when PCRE is built; if this is the case, the other behaviour can be + requested via the PCRE_BSR_UNICODE option. It is also possible to + specify these settings by starting a pattern string with one of the + following sequences: + + (*BSR_ANYCRLF) CR, LF, or CRLF only + (*BSR_UNICODE) any Unicode newline sequence + + These override the default and the options given to pcre_compile(), but + they can be overridden by options given to pcre_exec(). Note that these + special settings, which are not Perl-compatible, are recognized only at + the very start of a pattern, and that they must be in upper case. If + more than one of them is present, the last one is used. They can be + combined with a change of newline convention, for example, a pattern + can start with: + + (*ANY)(*BSR_ANYCRLF) + + Inside a character class, \R matches the letter "R". + + Unicode character properties + + When PCRE is built with Unicode character property support, three addi- + tional escape sequences that match characters with specific properties + are available. When not in UTF-8 mode, these sequences are of course + limited to testing characters whose codepoints are less than 256, but + they do work in this mode. The extra escape sequences are: + + \p{xx} a character with the xx property + \P{xx} a character without the xx property + \X an extended Unicode sequence + + The property names represented by xx above are limited to the Unicode + script names, the general category properties, and "Any", which matches + any character (including newline). Other properties such as "InMusical- + Symbols" are not currently supported by PCRE. Note that \P{Any} does + not match any characters, so always causes a match failure. + + Sets of Unicode characters are defined as belonging to certain scripts. + A character from one of these sets can be matched using a script name. + For example: + + \p{Greek} + \P{Han} + + Those that are not part of an identified script are lumped together as + "Common". The current list of scripts is: + + Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese, + Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform, + Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, + Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- + gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, + Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko, + Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician, + Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, + Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi. + + Each character has exactly one general category property, specified by + a two-letter abbreviation. For compatibility with Perl, negation can be + specified by including a circumflex between the opening brace and the + property name. For example, \p{^Lu} is the same as \P{Lu}. + + If only one letter is specified with \p or \P, it includes all the gen- + eral category properties that start with that letter. In this case, in + the absence of negation, the curly brackets in the escape sequence are + optional; these two examples have the same effect: + + \p{L} + \pL + + The following general category property codes are supported: + + C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate + + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter + + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark + + N Number + Nd Decimal number + Nl Letter number + No Other number + + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation + + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol + + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator + + The special property L& is also supported: it matches a character that + has the Lu, Ll, or Lt property, in other words, a letter that is not + classified as a modifier or "other". + + The Cs (Surrogate) property applies only to characters in the range + U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see + RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check- + ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in + the pcreapi page). + + The long synonyms for these properties that Perl supports (such as + \p{Letter}) are not supported by PCRE, nor is it permitted to prefix + any of these properties with "Is". + + No character that is in the Unicode table has the Cn (unassigned) prop- + erty. Instead, this property is assumed for any code point that is not + in the Unicode table. + + Specifying caseless matching does not affect these escape sequences. + For example, \p{Lu} always matches only upper case letters. + + The \X escape matches any number of Unicode characters that form an + extended Unicode sequence. \X is equivalent to + + (?>\PM\pM*) + + That is, it matches a character without the "mark" property, followed + by zero or more characters with the "mark" property, and treats the + sequence as an atomic group (see below). Characters with the "mark" + property are typically accents that affect the preceding character. + None of them have codepoints less than 256, so in non-UTF-8 mode \X + matches any one character. + + Matching characters by Unicode property is not fast, because PCRE has + to search a structure that contains data for over fifteen thousand + characters. That is why the traditional escape sequences such as \d and + \w do not use Unicode properties in PCRE. + + Resetting the match start + + The escape sequence \K, which is a Perl 5.10 feature, causes any previ- + ously matched characters not to be included in the final matched + sequence. For example, the pattern: + + foo\Kbar + + matches "foobar", but reports that it has matched "bar". This feature + is similar to a lookbehind assertion (described below). However, in + this case, the part of the subject before the real match does not have + to be of fixed length, as lookbehind assertions do. The use of \K does + not interfere with the setting of captured substrings. For example, + when the pattern + + (foo)\Kbar + + matches "foobar", the first substring is still set to "foo". + + Simple assertions + + The final use of backslash is for certain simple assertions. An asser- + tion specifies a condition that has to be met at a particular point in + a match, without consuming any characters from the subject string. The + use of subpatterns for more complicated assertions is described below. + The backslashed assertions are: + + \b matches at a word boundary + \B matches when not at a word boundary + \A matches at the start of the subject + \Z matches at the end of the subject + also matches before a newline at the end of the subject + \z matches only at the end of the subject + \G matches at the first matching position in the subject + + These assertions may not appear in character classes (but note that \b + has a different meaning, namely the backspace character, inside a char- + acter class). + + A word boundary is a position in the subject string where the current + character and the previous character do not both match \w or \W (i.e. + one matches \w and the other matches \W), or the start or end of the + string if the first or last character matches \w, respectively. + + The \A, \Z, and \z assertions differ from the traditional circumflex + and dollar (described in the next section) in that they only ever match + at the very start and end of the subject string, whatever options are + set. Thus, they are independent of multiline mode. These three asser- + tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which + affect only the behaviour of the circumflex and dollar metacharacters. + However, if the startoffset argument of pcre_exec() is non-zero, indi- + cating that matching is to start at a point other than the beginning of + the subject, \A can never match. The difference between \Z and \z is + that \Z matches before a newline at the end of the string as well as at + the very end, whereas \z matches only at the end. + + The \G assertion is true only when the current matching position is at + the start point of the match, as specified by the startoffset argument + of pcre_exec(). It differs from \A when the value of startoffset is + non-zero. By calling pcre_exec() multiple times with appropriate argu- + ments, you can mimic Perl's /g option, and it is in this kind of imple- + mentation where \G can be useful. + + Note, however, that PCRE's interpretation of \G, as the start of the + current match, is subtly different from Perl's, which defines it as the + end of the previous match. In Perl, these can be different when the + previously matched string was empty. Because PCRE does just one match + at a time, it cannot reproduce this behaviour. + + If all the alternatives of a pattern begin with \G, the expression is + anchored to the starting match position, and the "anchored" flag is set + in the compiled regular expression. + + +CIRCUMFLEX AND DOLLAR + + Outside a character class, in the default matching mode, the circumflex + character is an assertion that is true only if the current matching + point is at the start of the subject string. If the startoffset argu- + ment of pcre_exec() is non-zero, circumflex can never match if the + PCRE_MULTILINE option is unset. Inside a character class, circumflex + has an entirely different meaning (see below). + + Circumflex need not be the first character of the pattern if a number + of alternatives are involved, but it should be the first thing in each + alternative in which it appears if the pattern is ever to match that + branch. If all possible alternatives start with a circumflex, that is, + if the pattern is constrained to match only at the start of the sub- + ject, it is said to be an "anchored" pattern. (There are also other + constructs that can cause a pattern to be anchored.) + + A dollar character is an assertion that is true only if the current + matching point is at the end of the subject string, or immediately + before a newline at the end of the string (by default). Dollar need not + be the last character of the pattern if a number of alternatives are + involved, but it should be the last item in any branch in which it + appears. Dollar has no special meaning in a character class. + + The meaning of dollar can be changed so that it matches only at the + very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at + compile time. This does not affect the \Z assertion. + + The meanings of the circumflex and dollar characters are changed if the + PCRE_MULTILINE option is set. When this is the case, a circumflex + matches immediately after internal newlines as well as at the start of + the subject string. It does not match after a newline that ends the + string. A dollar matches before any newlines in the string, as well as + at the very end, when PCRE_MULTILINE is set. When newline is specified + as the two-character sequence CRLF, isolated CR and LF characters do + not indicate newlines. + + For example, the pattern /^abc$/ matches the subject string "def\nabc" + (where \n represents a newline) in multiline mode, but not otherwise. + Consequently, patterns that are anchored in single line mode because + all branches start with ^ are not anchored in multiline mode, and a + match for circumflex is possible when the startoffset argument of + pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if + PCRE_MULTILINE is set. + + Note that the sequences \A, \Z, and \z can be used to match the start + and end of the subject in both modes, and if all branches of a pattern + start with \A it is always anchored, whether or not PCRE_MULTILINE is + set. + + +FULL STOP (PERIOD, DOT) + + Outside a character class, a dot in the pattern matches any one charac- + ter in the subject string except (by default) a character that signi- + fies the end of a line. In UTF-8 mode, the matched character may be + more than one byte long. + + When a line ending is defined as a single character, dot never matches + that character; when the two-character sequence CRLF is used, dot does + not match CR if it is immediately followed by LF, but otherwise it + matches all characters (including isolated CRs and LFs). When any Uni- + code line endings are being recognized, dot does not match CR or LF or + any of the other line ending characters. + + The behaviour of dot with regard to newlines can be changed. If the + PCRE_DOTALL option is set, a dot matches any one character, without + exception. If the two-character sequence CRLF is present in the subject + string, it takes two dots to match it. + + The handling of dot is entirely independent of the handling of circum- + flex and dollar, the only relationship being that they both involve + newlines. Dot has no special meaning in a character class. + + +MATCHING A SINGLE BYTE + + Outside a character class, the escape sequence \C matches any one byte, + both in and out of UTF-8 mode. Unlike a dot, it always matches any + line-ending characters. The feature is provided in Perl in order to + match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char- + acters into individual bytes, what remains in the string may be a mal- + formed UTF-8 string. For this reason, the \C escape sequence is best + avoided. + + PCRE does not allow \C to appear in lookbehind assertions (described + below), because in UTF-8 mode this would make it impossible to calcu- + late the length of the lookbehind. + + +SQUARE BRACKETS AND CHARACTER CLASSES + + An opening square bracket introduces a character class, terminated by a + closing square bracket. A closing square bracket on its own is not spe- + cial. If a closing square bracket is required as a member of the class, + it should be the first data character in the class (after an initial + circumflex, if present) or escaped with a backslash. + + A character class matches a single character in the subject. In UTF-8 + mode, the character may occupy more than one byte. A matched character + must be in the set of characters defined by the class, unless the first + character in the class definition is a circumflex, in which case the + subject character must not be in the set defined by the class. If a + circumflex is actually required as a member of the class, ensure it is + not the first character, or escape it with a backslash. + + For example, the character class [aeiou] matches any lower case vowel, + while [^aeiou] matches any character that is not a lower case vowel. + Note that a circumflex is just a convenient notation for specifying the + characters that are in the class by enumerating those that are not. A + class that starts with a circumflex is not an assertion: it still con- + sumes a character from the subject string, and therefore it fails if + the current pointer is at the end of the string. + + In UTF-8 mode, characters with values greater than 255 can be included + in a class as a literal string of bytes, or by using the \x{ escaping + mechanism. + + When caseless matching is set, any letters in a class represent both + their upper case and lower case versions, so for example, a caseless + [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not + match "A", whereas a caseful version would. In UTF-8 mode, PCRE always + understands the concept of case for characters whose values are less + than 128, so caseless matching is always possible. For characters with + higher values, the concept of case is supported if PCRE is compiled + with Unicode property support, but not otherwise. If you want to use + caseless matching for characters 128 and above, you must ensure that + PCRE is compiled with Unicode property support as well as with UTF-8 + support. + + Characters that might indicate line breaks are never treated in any + special way when matching character classes, whatever line-ending + sequence is in use, and whatever setting of the PCRE_DOTALL and + PCRE_MULTILINE options is used. A class such as [^a] always matches one + of these characters. + + The minus (hyphen) character can be used to specify a range of charac- + ters in a character class. For example, [d-m] matches any letter + between d and m, inclusive. If a minus character is required in a + class, it must be escaped with a backslash or appear in a position + where it cannot be interpreted as indicating a range, typically as the + first or last character in the class. + + It is not possible to have the literal character "]" as the end charac- + ter of a range. A pattern such as [W-]46] is interpreted as a class of + two characters ("W" and "-") followed by a literal string "46]", so it + would match "W46]" or "-46]". However, if the "]" is escaped with a + backslash it is interpreted as the end of range, so [W-\]46] is inter- + preted as a class containing a range followed by two other characters. + The octal or hexadecimal representation of "]" can also be used to end + a range. + + Ranges operate in the collating sequence of character values. They can + also be used for characters specified numerically, for example + [\000-\037]. In UTF-8 mode, ranges can include characters whose values + are greater than 255, for example [\x{100}-\x{2ff}]. + + If a range that includes letters is used when caseless matching is set, + it matches the letters in either case. For example, [W-c] is equivalent + to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if + character tables for a French locale are in use, [\xc8-\xcb] matches + accented E characters in both cases. In UTF-8 mode, PCRE supports the + concept of case for characters with values greater than 128 only when + it is compiled with Unicode property support. + + The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear + in a character class, and add the characters that they match to the + class. For example, [\dABCDEF] matches any hexadecimal digit. A circum- + flex can conveniently be used with the upper case character types to + specify a more restricted set of characters than the matching lower + case type. For example, the class [^\W_] matches any letter or digit, + but not underscore. + + The only metacharacters that are recognized in character classes are + backslash, hyphen (only where it can be interpreted as specifying a + range), circumflex (only at the start), opening square bracket (only + when it can be interpreted as introducing a POSIX class name - see the + next section), and the terminating closing square bracket. However, + escaping other non-alphanumeric characters does no harm. + + +POSIX CHARACTER CLASSES + + Perl supports the POSIX notation for character classes. This uses names + enclosed by [: and :] within the enclosing square brackets. PCRE also + supports this notation. For example, + + [01[:alpha:]%] + + matches "0", "1", any alphabetic character, or "%". The supported class + names are + + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + blank space or tab only + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (not quite the same as \s) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits + + The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), + and space (32). Notice that this list includes the VT character (code + 11). This makes "space" different to \s, which does not include VT (for + Perl compatibility). + + The name "word" is a Perl extension, and "blank" is a GNU extension + from Perl 5.8. Another Perl extension is negation, which is indicated + by a ^ character after the colon. For example, + + [12[:^digit:]] + + matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the + POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but + these are not supported, and an error is given if they are encountered. + + In UTF-8 mode, characters with values greater than 128 do not match any + of the POSIX character classes. + + +VERTICAL BAR + + Vertical bar characters are used to separate alternative patterns. For + example, the pattern + + gilbert|sullivan + + matches either "gilbert" or "sullivan". Any number of alternatives may + appear, and an empty alternative is permitted (matching the empty + string). The matching process tries each alternative in turn, from left + to right, and the first one that succeeds is used. If the alternatives + are within a subpattern (defined below), "succeeds" means matching the + rest of the main pattern as well as the alternative in the subpattern. + + +INTERNAL OPTION SETTING + + The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and + PCRE_EXTENDED options (which are Perl-compatible) can be changed from + within the pattern by a sequence of Perl option letters enclosed + between "(?" and ")". The option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + + For example, (?im) sets caseless, multiline matching. It is also possi- + ble to unset these options by preceding the letter with a hyphen, and a + combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- + LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, + is also permitted. If a letter appears both before and after the + hyphen, the option is unset. + + The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA + can be changed in the same way as the Perl-compatible options by using + the characters J, U and X respectively. + + When an option change occurs at top level (that is, not inside subpat- + tern parentheses), the change applies to the remainder of the pattern + that follows. If the change is placed right at the start of a pattern, + PCRE extracts it into the global options (and it will therefore show up + in data extracted by the pcre_fullinfo() function). + + An option change within a subpattern (see below for a description of + subpatterns) affects only that part of the current pattern that follows + it, so + + (a(?i)b)c + + matches abc and aBc and no other strings (assuming PCRE_CASELESS is not + used). By this means, options can be made to have different settings + in different parts of the pattern. Any changes made in one alternative + do carry on into subsequent branches within the same subpattern. For + example, + + (a(?i)b|c) + + matches "ab", "aB", "c", and "C", even though when matching "C" the + first branch is abandoned before the option setting. This is because + the effects of option settings happen at compile time. There would be + some very weird behaviour otherwise. + + Note: There are other PCRE-specific options that can be set by the + application when the compile or match functions are called. In some + cases the pattern can contain special leading sequences to override + what the application has set or what has been defaulted. Details are + given in the section entitled "Newline sequences" above. + + +SUBPATTERNS + + Subpatterns are delimited by parentheses (round brackets), which can be + nested. Turning part of a pattern into a subpattern does two things: + + 1. It localizes a set of alternatives. For example, the pattern + + cat(aract|erpillar|) + + matches one of the words "cat", "cataract", or "caterpillar". Without + the parentheses, it would match "cataract", "erpillar" or an empty + string. + + 2. It sets up the subpattern as a capturing subpattern. This means + that, when the whole pattern matches, that portion of the subject + string that matched the subpattern is passed back to the caller via the + ovector argument of pcre_exec(). Opening parentheses are counted from + left to right (starting from 1) to obtain numbers for the capturing + subpatterns. + + For example, if the string "the red king" is matched against the pat- + tern + + the ((red|white) (king|queen)) + + the captured substrings are "red king", "red", and "king", and are num- + bered 1, 2, and 3, respectively. + + The fact that plain parentheses fulfil two functions is not always + helpful. There are often times when a grouping subpattern is required + without a capturing requirement. If an opening parenthesis is followed + by a question mark and a colon, the subpattern does not do any captur- + ing, and is not counted when computing the number of any subsequent + capturing subpatterns. For example, if the string "the white queen" is + matched against the pattern + + the ((?:red|white) (king|queen)) + + the captured substrings are "white queen" and "queen", and are numbered + 1 and 2. The maximum number of capturing subpatterns is 65535. + + As a convenient shorthand, if any option settings are required at the + start of a non-capturing subpattern, the option letters may appear + between the "?" and the ":". Thus the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + + match exactly the same set of strings. Because alternative branches are + tried from left to right, and options are not reset until the end of + the subpattern is reached, an option setting in one branch does affect + subsequent branches, so the above patterns match "SUNDAY" as well as + "Saturday". + + +DUPLICATE SUBPATTERN NUMBERS + + Perl 5.10 introduced a feature whereby each alternative in a subpattern + uses the same numbers for its capturing parentheses. Such a subpattern + starts with (?| and is itself a non-capturing subpattern. For example, + consider this pattern: + + (?|(Sat)ur|(Sun))day + + Because the two alternatives are inside a (?| group, both sets of cap- + turing parentheses are numbered one. Thus, when the pattern matches, + you can look at captured substring number one, whichever alternative + matched. This construct is useful when you want to capture part, but + not all, of one of a number of alternatives. Inside a (?| group, paren- + theses are numbered as usual, but the number is reset at the start of + each branch. The numbers of any capturing buffers that follow the sub- + pattern start after the highest number used in any branch. The follow- + ing example is taken from the Perl documentation. The numbers under- + neath show in which buffer the captured content will be stored. + + # before ---------------branch-reset----------- after + / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x + # 1 2 2 3 2 3 4 + + A backreference or a recursive call to a numbered subpattern always + refers to the first one in the pattern with the given number. + + An alternative approach to using this "branch reset" feature is to use + duplicate named subpatterns, as described in the next section. + + +NAMED SUBPATTERNS + + Identifying capturing parentheses by number is simple, but it can be + very hard to keep track of the numbers in complicated regular expres- + sions. Furthermore, if an expression is modified, the numbers may + change. To help with this difficulty, PCRE supports the naming of sub- + patterns. This feature was not added to Perl until release 5.10. Python + had the feature earlier, and PCRE introduced it at release 4.0, using + the Python syntax. PCRE now supports both the Perl and the Python syn- + tax. + + In PCRE, a subpattern can be named in one of three ways: (?...) + or (?'name'...) as in Perl, or (?P...) as in Python. References + to capturing parentheses from other parts of the pattern, such as back- + references, recursion, and conditions, can be made by name as well as + by number. + + Names consist of up to 32 alphanumeric characters and underscores. + Named capturing parentheses are still allocated numbers as well as + names, exactly as if the names were not present. The PCRE API provides + function calls for extracting the name-to-number translation table from + a compiled pattern. There is also a convenience function for extracting + a captured substring by name. + + By default, a name must be unique within a pattern, but it is possible + to relax this constraint by setting the PCRE_DUPNAMES option at compile + time. This can be useful for patterns where only one instance of the + named parentheses can match. Suppose you want to match the name of a + weekday, either as a 3-letter abbreviation or as the full name, and in + both cases you want to extract the abbreviation. This pattern (ignoring + the line breaks) does the job: + + (?Mon|Fri|Sun)(?:day)?| + (?Tue)(?:sday)?| + (?Wed)(?:nesday)?| + (?Thu)(?:rsday)?| + (?Sat)(?:urday)? + + There are five capturing substrings, but only one is ever set after a + match. (An alternative way of solving this problem is to use a "branch + reset" subpattern, as described in the previous section.) + + The convenience function for extracting the data by name returns the + substring for the first (and in this example, the only) subpattern of + that name that matched. This saves searching to find which numbered + subpattern it was. If you make a reference to a non-unique named sub- + pattern from elsewhere in the pattern, the one that corresponds to the + lowest number is used. For further details of the interfaces for han- + dling named subpatterns, see the pcreapi documentation. + + +REPETITION + + Repetition is specified by quantifiers, which can follow any of the + following items: + + a literal data character + the dot metacharacter + the \C escape sequence + the \X escape sequence (in UTF-8 mode with Unicode properties) + the \R escape sequence + an escape such as \d that matches a single character + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion) + + The general repetition quantifier specifies a minimum and maximum num- + ber of permitted matches, by giving the two numbers in curly brackets + (braces), separated by a comma. The numbers must be less than 65536, + and the first must be less than or equal to the second. For example: + + z{2,4} + + matches "zz", "zzz", or "zzzz". A closing brace on its own is not a + special character. If the second number is omitted, but the comma is + present, there is no upper limit; if the second number and the comma + are both omitted, the quantifier specifies an exact number of required + matches. Thus + + [aeiou]{3,} + + matches at least 3 successive vowels, but may match many more, while + + \d{8} + + matches exactly 8 digits. An opening curly bracket that appears in a + position where a quantifier is not allowed, or one that does not match + the syntax of a quantifier, is taken as a literal character. For exam- + ple, {,6} is not a quantifier, but a literal string of four characters. + + In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to + individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- + acters, each of which is represented by a two-byte sequence. Similarly, + when Unicode property support is available, \X{3} matches three Unicode + extended sequences, each of which may be several bytes long (and they + may be of different lengths). + + The quantifier {0} is permitted, causing the expression to behave as if + the previous item and the quantifier were not present. This may be use- + ful for subpatterns that are referenced as subroutines from elsewhere + in the pattern. Items other than subpatterns that have a {0} quantifier + are omitted from the compiled pattern. + + For convenience, the three most common quantifiers have single-charac- + ter abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + + It is possible to construct infinite loops by following a subpattern + that can match no characters with a quantifier that has no upper limit, + for example: + + (a?)* + + Earlier versions of Perl and PCRE used to give an error at compile time + for such patterns. However, because there are cases where this can be + useful, such patterns are now accepted, but if any repetition of the + subpattern does in fact match no characters, the loop is forcibly bro- + ken. + + By default, the quantifiers are "greedy", that is, they match as much + as possible (up to the maximum number of permitted times), without + causing the rest of the pattern to fail. The classic example of where + this gives problems is in trying to match comments in C programs. These + appear between /* and */ and within the comment, individual * and / + characters may appear. An attempt to match C comments by applying the + pattern + + /\*.*\*/ + + to the string + + /* first comment */ not comment /* second comment */ + + fails, because it matches the entire string owing to the greediness of + the .* item. + + However, if a quantifier is followed by a question mark, it ceases to + be greedy, and instead matches the minimum number of times possible, so + the pattern + + /\*.*?\*/ + + does the right thing with the C comments. The meaning of the various + quantifiers is not otherwise changed, just the preferred number of + matches. Do not confuse this use of question mark with its use as a + quantifier in its own right. Because it has two uses, it can sometimes + appear doubled, as in + + \d??\d + + which matches one digit by preference, but can match two if that is the + only way the rest of the pattern matches. + + If the PCRE_UNGREEDY option is set (an option that is not available in + Perl), the quantifiers are not greedy by default, but individual ones + can be made greedy by following them with a question mark. In other + words, it inverts the default behaviour. + + When a parenthesized subpattern is quantified with a minimum repeat + count that is greater than 1 or with a limited maximum, more memory is + required for the compiled pattern, in proportion to the size of the + minimum or maximum. + + If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- + alent to Perl's /s) is set, thus allowing the dot to match newlines, + the pattern is implicitly anchored, because whatever follows will be + tried against every character position in the subject string, so there + is no point in retrying the overall match at any position after the + first. PCRE normally treats such a pattern as though it were preceded + by \A. + + In cases where it is known that the subject string contains no new- + lines, it is worth setting PCRE_DOTALL in order to obtain this opti- + mization, or alternatively using ^ to indicate anchoring explicitly. + + However, there is one situation where the optimization cannot be used. + When .* is inside capturing parentheses that are the subject of a + backreference elsewhere in the pattern, a match at the start may fail + where a later one succeeds. Consider, for example: + + (.*)abc\1 + + If the subject is "xyz123abc123" the match point is the fourth charac- + ter. For this reason, such a pattern is not implicitly anchored. + + When a capturing subpattern is repeated, the value captured is the sub- + string that matched the final iteration. For example, after + + (tweedle[dume]{3}\s*)+ + + has matched "tweedledum tweedledee" the value of the captured substring + is "tweedledee". However, if there are nested capturing subpatterns, + the corresponding captured values may have been set in previous itera- + tions. For example, after + + /(a|(b))+/ + + matches "aba" the value of the second captured substring is "b". + + +ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS + + With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") + repetition, failure of what follows normally causes the repeated item + to be re-evaluated to see if a different number of repeats allows the + rest of the pattern to match. Sometimes it is useful to prevent this, + either to change the nature of the match, or to cause it fail earlier + than it otherwise might, when the author of the pattern knows there is + no point in carrying on. + + Consider, for example, the pattern \d+foo when applied to the subject + line + + 123456bar + + After matching all 6 digits and then failing to match "foo", the normal + action of the matcher is to try again with only 5 digits matching the + \d+ item, and then with 4, and so on, before ultimately failing. + "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides + the means for specifying that once a subpattern has matched, it is not + to be re-evaluated in this way. + + If we use atomic grouping for the previous example, the matcher gives + up immediately on failing to match "foo" the first time. The notation + is a kind of special parenthesis, starting with (?> as in this example: + + (?>\d+)foo + + This kind of parenthesis "locks up" the part of the pattern it con- + tains once it has matched, and a failure further into the pattern is + prevented from backtracking into it. Backtracking past it to previous + items, however, works as normal. + + An alternative description is that a subpattern of this type matches + the string of characters that an identical standalone pattern would + match, if anchored at the current point in the subject string. + + Atomic grouping subpatterns are not capturing subpatterns. Simple cases + such as the above example can be thought of as a maximizing repeat that + must swallow everything it can. So, while both \d+ and \d+? are pre- + pared to adjust the number of digits they match in order to make the + rest of the pattern match, (?>\d+) can only match an entire sequence of + digits. + + Atomic groups in general can of course contain arbitrarily complicated + subpatterns, and can be nested. However, when the subpattern for an + atomic group is just a single repeated item, as in the example above, a + simpler notation, called a "possessive quantifier" can be used. This + consists of an additional + character following a quantifier. Using + this notation, the previous example can be rewritten as + + \d++foo + + Note that a possessive quantifier can be used with an entire group, for + example: + + (abc|xyz){2,3}+ + + Possessive quantifiers are always greedy; the setting of the + PCRE_UNGREEDY option is ignored. They are a convenient notation for the + simpler forms of atomic group. However, there is no difference in the + meaning of a possessive quantifier and the equivalent atomic group, + though there may be a performance difference; possessive quantifiers + should be slightly faster. + + The possessive quantifier syntax is an extension to the Perl 5.8 syn- + tax. Jeffrey Friedl originated the idea (and the name) in the first + edition of his book. Mike McCloskey liked it, so implemented it when he + built Sun's Java package, and PCRE copied it from there. It ultimately + found its way into Perl at release 5.10. + + PCRE has an optimization that automatically "possessifies" certain sim- + ple pattern constructs. For example, the sequence A+B is treated as + A++B because there is no point in backtracking into a sequence of A's + when B must follow. + + When a pattern contains an unlimited repeat inside a subpattern that + can itself be repeated an unlimited number of times, the use of an + atomic group is the only way to avoid some failing matches taking a + very long time indeed. The pattern + + (\D+|<\d+>)*[!?] + + matches an unlimited number of substrings that either consist of non- + digits, or digits enclosed in <>, followed by either ! or ?. When it + matches, it runs quickly. However, if it is applied to + + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + + it takes a long time before reporting failure. This is because the + string can be divided between the internal \D+ repeat and the external + * repeat in a large number of ways, and all have to be tried. (The + example uses [!?] rather than a single character at the end, because + both PCRE and Perl have an optimization that allows for fast failure + when a single character is used. They remember the last single charac- + ter that is required for a match, and fail early if it is not present + in the string.) If the pattern is changed so that it uses an atomic + group, like this: + + ((?>\D+)|<\d+>)*[!?] + + sequences of non-digits cannot be broken, and failure happens quickly. + + +BACK REFERENCES + + Outside a character class, a backslash followed by a digit greater than + 0 (and possibly further digits) is a back reference to a capturing sub- + pattern earlier (that is, to its left) in the pattern, provided there + have been that many previous capturing left parentheses. + + However, if the decimal number following the backslash is less than 10, + it is always taken as a back reference, and causes an error only if + there are not that many capturing left parentheses in the entire pat- + tern. In other words, the parentheses that are referenced need not be + to the left of the reference for numbers less than 10. A "forward back + reference" of this type can make sense when a repetition is involved + and the subpattern to the right has participated in an earlier itera- + tion. + + It is not possible to have a numerical "forward back reference" to a + subpattern whose number is 10 or more using this syntax because a + sequence such as \50 is interpreted as a character defined in octal. + See the subsection entitled "Non-printing characters" above for further + details of the handling of digits following a backslash. There is no + such problem when named parentheses are used. A back reference to any + subpattern is possible using named parentheses (see below). + + Another way of avoiding the ambiguity inherent in the use of digits + following a backslash is to use the \g escape sequence, which is a fea- + ture introduced in Perl 5.10. This escape must be followed by an + unsigned number or a negative number, optionally enclosed in braces. + These examples are all identical: + + (ring), \1 + (ring), \g1 + (ring), \g{1} + + An unsigned number specifies an absolute reference without the ambigu- + ity that is present in the older syntax. It is also useful when literal + digits follow the reference. A negative number is a relative reference. + Consider this example: + + (abc(def)ghi)\g{-1} + + The sequence \g{-1} is a reference to the most recently started captur- + ing subpattern before \g, that is, is it equivalent to \2. Similarly, + \g{-2} would be equivalent to \1. The use of relative references can be + helpful in long patterns, and also in patterns that are created by + joining together fragments that contain references within themselves. + + A back reference matches whatever actually matched the capturing sub- + pattern in the current subject string, rather than anything matching + the subpattern itself (see "Subpatterns as subroutines" below for a way + of doing that). So the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsibility", but + not "sense and responsibility". If caseful matching is in force at the + time of the back reference, the case of letters is relevant. For exam- + ple, + + ((?i)rah)\s+\1 + + matches "rah rah" and "RAH RAH", but not "RAH rah", even though the + original capturing subpattern is matched caselessly. + + There are several different ways of writing back references to named + subpatterns. The .NET syntax \k{name} and the Perl syntax \k or + \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's + unified back reference syntax, in which \g can be used for both numeric + and named references, is also supported. We could rewrite the above + example in any of the following ways: + + (?(?i)rah)\s+\k + (?'p1'(?i)rah)\s+\k{p1} + (?P(?i)rah)\s+(?P=p1) + (?(?i)rah)\s+\g{p1} + + A subpattern that is referenced by name may appear in the pattern + before or after the reference. + + There may be more than one back reference to the same subpattern. If a + subpattern has not actually been used in a particular match, any back + references to it always fail. For example, the pattern + + (a|(bc))\2 + + always fails if it starts to match "a" rather than "bc". Because there + may be many capturing parentheses in a pattern, all digits following + the backslash are taken as part of a potential back reference number. + If the pattern continues with a digit character, some delimiter must be + used to terminate the back reference. If the PCRE_EXTENDED option is + set, this can be whitespace. Otherwise an empty comment (see "Com- + ments" below) can be used. + + A back reference that occurs inside the parentheses to which it refers + fails when the subpattern is first used, so, for example, (a\1) never + matches. However, such references can be useful inside repeated sub- + patterns. For example, the pattern + + (a|b\1)+ + + matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- + ation of the subpattern, the back reference matches the character + string corresponding to the previous iteration. In order for this to + work, the pattern must be such that the first iteration does not need + to match the back reference. This can be done using alternation, as in + the example above, or by a quantifier with a minimum of zero. + + +ASSERTIONS + + An assertion is a test on the characters following or preceding the + current matching point that does not actually consume any characters. + The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are + described above. + + More complicated assertions are coded as subpatterns. There are two + kinds: those that look ahead of the current position in the subject + string, and those that look behind it. An assertion subpattern is + matched in the normal way, except that it does not cause the current + matching position to be changed. + + Assertion subpatterns are not capturing subpatterns, and may not be + repeated, because it makes no sense to assert the same thing several + times. If any kind of assertion contains capturing subpatterns within + it, these are counted for the purposes of numbering the capturing sub- + patterns in the whole pattern. However, substring capturing is carried + out only for positive assertions, because it does not make sense for + negative assertions. + + Lookahead assertions + + Lookahead assertions start with (?= for positive assertions and (?! for + negative assertions. For example, + + \w+(?=;) + + matches a word followed by a semicolon, but does not include the semi- + colon in the match, and + + foo(?!bar) + + matches any occurrence of "foo" that is not followed by "bar". Note + that the apparently similar pattern + + (?!foo)bar + + does not find an occurrence of "bar" that is preceded by something + other than "foo"; it finds any occurrence of "bar" whatsoever, because + the assertion (?!foo) is always true when the next three characters are + "bar". A lookbehind assertion is needed to achieve the other effect. + + If you want to force a matching failure at some point in a pattern, the + most convenient way to do it is with (?!) because an empty string + always matches, so an assertion that requires there not to be an empty + string must always fail. + + Lookbehind assertions + + Lookbehind assertions start with (?<= for positive assertions and (?)...) or (?('name')...) to test for a + used subpattern by name. For compatibility with earlier versions of + PCRE, which had this facility before Perl, the syntax (?(name)...) is + also recognized. However, there is a possible ambiguity with this syn- + tax, because subpattern names may consist entirely of digits. PCRE + looks first for a named subpattern; if it cannot find one and the name + consists entirely of digits, PCRE looks for a subpattern of that num- + ber, which must be greater than zero. Using subpattern names that con- + sist entirely of digits is not recommended. + + Rewriting the above example to use a named subpattern gives this: + + (? \( )? [^()]+ (?() \) ) + + + Checking for pattern recursion + + If the condition is the string (R), and there is no subpattern with the + name R, the condition is true if a recursive call to the whole pattern + or any subpattern has been made. If digits or a name preceded by amper- + sand follow the letter R, for example: + + (?(R3)...) or (?(R&name)...) + + the condition is true if the most recent recursion is into the subpat- + tern whose number or name is given. This condition does not check the + entire recursion stack. + + At "top level", all these recursion test conditions are false. Recur- + sive patterns are described below. + + Defining subpatterns for use by reference only + + If the condition is the string (DEFINE), and there is no subpattern + with the name DEFINE, the condition is always false. In this case, + there may be only one alternative in the subpattern. It is always + skipped if control reaches this point in the pattern; the idea of + DEFINE is that it can be used to define "subroutines" that can be ref- + erenced from elsewhere. (The use of "subroutines" is described below.) + For example, a pattern to match an IPv4 address could be written like + this (ignore whitespace and line breaks): + + (?(DEFINE) (? 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) + \b (?&byte) (\.(?&byte)){3} \b + + The first part of the pattern is a DEFINE group inside which a another + group named "byte" is defined. This matches an individual component of + an IPv4 address (a number less than 256). When matching takes place, + this part of the pattern is skipped because DEFINE acts like a false + condition. + + The rest of the pattern uses references to the named group to match the + four dot-separated components of an IPv4 address, insisting on a word + boundary at each end. + + Assertion conditions + + If the condition is not in any of the above formats, it must be an + assertion. This may be a positive or negative lookahead or lookbehind + assertion. Consider this pattern, again containing non-significant + white space, and with the two alternatives on the second line: + + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) + + The condition is a positive lookahead assertion that matches an + optional sequence of non-letters followed by a letter. In other words, + it tests for the presence of at least one letter in the subject. If a + letter is found, the subject is matched against the first alternative; + otherwise it is matched against the second. This pattern matches + strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + letters and dd are digits. + + +COMMENTS + + The sequence (?# marks the start of a comment that continues up to the + next closing parenthesis. Nested parentheses are not permitted. The + characters that make up a comment play no part in the pattern matching + at all. + + If the PCRE_EXTENDED option is set, an unescaped # character outside a + character class introduces a comment that continues to immediately + after the next newline in the pattern. + + +RECURSIVE PATTERNS + + Consider the problem of matching a string in parentheses, allowing for + unlimited nested parentheses. Without the use of recursion, the best + that can be done is to use a pattern that matches up to some fixed + depth of nesting. It is not possible to handle an arbitrary nesting + depth. + + For some time, Perl has provided a facility that allows regular expres- + sions to recurse (amongst other things). It does this by interpolating + Perl code in the expression at run time, and the code can refer to the + expression itself. A Perl pattern using code interpolation to solve the + parentheses problem can be created like this: + + $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; + + The (?p{...}) item interpolates Perl code at run time, and in this case + refers recursively to the pattern in which it appears. + + Obviously, PCRE cannot support the interpolation of Perl code. Instead, + it supports special syntax for recursion of the entire pattern, and + also for individual subpattern recursion. After its introduction in + PCRE and Python, this kind of recursion was introduced into Perl at + release 5.10. + + A special item that consists of (? followed by a number greater than + zero and a closing parenthesis is a recursive call of the subpattern of + the given number, provided that it occurs inside that subpattern. (If + not, it is a "subroutine" call, which is described in the next sec- + tion.) The special item (?R) or (?0) is a recursive call of the entire + regular expression. + + In PCRE (like Python, but unlike Perl), a recursive subpattern call is + always treated as an atomic group. That is, once it has matched some of + the subject string, it is never re-entered, even if it contains untried + alternatives and there is a subsequent matching failure. + + This PCRE pattern solves the nested parentheses problem (assume the + PCRE_EXTENDED option is set so that white space is ignored): + + \( ( (?>[^()]+) | (?R) )* \) + + First it matches an opening parenthesis. Then it matches any number of + substrings which can either be a sequence of non-parentheses, or a + recursive match of the pattern itself (that is, a correctly parenthe- + sized substring). Finally there is a closing parenthesis. + + If this were part of a larger pattern, you would not want to recurse + the entire pattern, so instead you could use this: + + ( \( ( (?>[^()]+) | (?1) )* \) ) + + We have put the pattern into parentheses, and caused the recursion to + refer to them instead of the whole pattern. + + In a larger pattern, keeping track of parenthesis numbers can be + tricky. This is made easier by the use of relative references. (A Perl + 5.10 feature.) Instead of (?1) in the pattern above you can write + (?-2) to refer to the second most recently opened parentheses preceding + the recursion. In other words, a negative number counts capturing + parentheses leftwards from the point at which it is encountered. + + It is also possible to refer to subsequently opened parentheses, by + writing references such as (?+2). However, these cannot be recursive + because the reference is not inside the parentheses that are refer- + enced. They are always "subroutine" calls, as described in the next + section. + + An alternative approach is to use named parentheses instead. The Perl + syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also + supported. We could rewrite the above example as follows: + + (? \( ( (?>[^()]+) | (?&pn) )* \) ) + + If there is more than one subpattern with the same name, the earliest + one is used. + + This particular example pattern that we have been looking at contains + nested unlimited repeats, and so the use of atomic grouping for match- + ing strings of non-parentheses is important when applying the pattern + to strings that do not match. For example, when this pattern is applied + to + + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() + + it yields "no match" quickly. However, if atomic grouping is not used, + the match runs for a very long time indeed because there are so many + different ways the + and * repeats can carve up the subject, and all + have to be tested before failure can be reported. + + At the end of a match, the values set for any capturing subpatterns are + those from the outermost level of the recursion at which the subpattern + value is set. If you want to obtain intermediate values, a callout + function can be used (see below and the pcrecallout documentation). If + the pattern above is matched against + + (ab(cd)ef) + + the value for the capturing parentheses is "ef", which is the last + value taken on at the top level. If additional parentheses are added, + giving + + \( ( ( (?>[^()]+) | (?R) )* ) \) + ^ ^ + ^ ^ + + the string they capture is "ab(cd)ef", the contents of the top level + parentheses. If there are more than 15 capturing parentheses in a pat- + tern, PCRE has to obtain extra memory to store data during a recursion, + which it does by using pcre_malloc, freeing it via pcre_free after- + wards. If no memory can be obtained, the match fails with the + PCRE_ERROR_NOMEMORY error. + + Do not confuse the (?R) item with the condition (R), which tests for + recursion. Consider this pattern, which matches text in angle brack- + ets, allowing for arbitrary nesting. Only digits are allowed in nested + brackets (that is, when recursing), whereas any characters are permit- + ted at the outer level. + + < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > + + In this pattern, (?(R) is the start of a conditional subpattern, with + two different alternatives for the recursive and non-recursive cases. + The (?R) item is the actual recursive call. + + +SUBPATTERNS AS SUBROUTINES + + If the syntax for a recursive subpattern reference (either by number or + by name) is used outside the parentheses to which it refers, it oper- + ates like a subroutine in a programming language. The "called" subpat- + tern may be defined before or after the reference. A numbered reference + can be absolute or relative, as in these examples: + + (...(absolute)...)...(?2)... + (...(relative)...)...(?-1)... + (...(?+1)...(relative)... + + An earlier example pointed out that the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsibility", but + not "sense and responsibility". If instead the pattern + + (sens|respons)e and (?1)ibility + + is used, it does match "sense and responsibility" as well as the other + two strings. Another example is given in the discussion of DEFINE + above. + + Like recursive subpatterns, a "subroutine" call is always treated as an + atomic group. That is, once it has matched some of the subject string, + it is never re-entered, even if it contains untried alternatives and + there is a subsequent matching failure. + + When a subpattern is used as a subroutine, processing options such as + case-independence are fixed when the subpattern is defined. They cannot + be changed for different calls. For example, consider this pattern: + + (abc)(?i:(?-1)) + + It matches "abcabc". It does not match "abcABC" because the change of + processing option does not affect the called subpattern. + + +ONIGURUMA SUBROUTINE SYNTAX + + For compatibility with Oniguruma, the non-Perl syntax \g followed by a + name or a number enclosed either in angle brackets or single quotes, is + an alternative syntax for referencing a subpattern as a subroutine, + possibly recursively. Here are two of the examples used above, rewrit- + ten using this syntax: + + (? \( ( (?>[^()]+) | \g )* \) ) + (sens|respons)e and \g'1'ibility + + PCRE supports an extension to Oniguruma: if a number is preceded by a + plus or a minus sign it is taken as a relative reference. For example: + + (abc)(?i:\g<-1>) + + Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not + synonymous. The former is a back reference; the latter is a subroutine + call. + + +CALLOUTS + + Perl has a feature whereby using the sequence (?{...}) causes arbitrary + Perl code to be obeyed in the middle of matching a regular expression. + This makes it possible, amongst other things, to extract different sub- + strings that match the same pair of parentheses when there is a repeti- + tion. + + PCRE provides a similar feature, but of course it cannot obey arbitrary + Perl code. The feature is called "callout". The caller of PCRE provides + an external function by putting its entry point in the global variable + pcre_callout. By default, this variable contains NULL, which disables + all calling out. + + Within a regular expression, (?C) indicates the points at which the + external function is to be called. If you want to identify different + callout points, you can put a number less than 256 after the letter C. + The default value is zero. For example, this pattern has two callout + points: + + (?C1)abc(?C2)def + + If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are + automatically installed before each item in the pattern. They are all + numbered 255. + + During matching, when PCRE reaches a callout point (and pcre_callout is + set), the external function is called. It is provided with the number + of the callout, the position in the pattern, and, optionally, one item + of data originally supplied by the caller of pcre_exec(). The callout + function may cause matching to proceed, to backtrack, or to fail alto- + gether. A complete description of the interface to the callout function + is given in the pcrecallout documentation. + + +BACKTRACKING CONTROL + + Perl 5.10 introduced a number of "Special Backtracking Control Verbs", + which are described in the Perl documentation as "experimental and sub- + ject to change or removal in a future version of Perl". It goes on to + say: "Their usage in production code should be noted to avoid problems + during upgrades." The same remarks apply to the PCRE features described + in this section. + + Since these verbs are specifically related to backtracking, most of + them can be used only when the pattern is to be matched using + pcre_exec(), which uses a backtracking algorithm. With the exception of + (*FAIL), which behaves like a failing negative assertion, they cause an + error if encountered by pcre_dfa_exec(). + + The new verbs make use of what was previously invalid syntax: an open- + ing parenthesis followed by an asterisk. In Perl, they are generally of + the form (*VERB:ARG) but PCRE does not support the use of arguments, so + its general form is just (*VERB). Any number of these verbs may occur + in a pattern. There are two kinds: + + Verbs that act immediately + + The following verbs act as soon as they are encountered: + + (*ACCEPT) + + This verb causes the match to end successfully, skipping the remainder + of the pattern. When inside a recursion, only the innermost pattern is + ended immediately. PCRE differs from Perl in what happens if the + (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is + captured: in PCRE no data is captured. For example: + + A(A|B(*ACCEPT)|C)D + + This matches "AB", "AAD", or "ACD", but when it matches "AB", no data + is captured. + + (*FAIL) or (*F) + + This verb causes the match to fail, forcing backtracking to occur. It + is equivalent to (?!) but easier to read. The Perl documentation notes + that it is probably useful only when combined with (?{}) or (??{}). + Those are, of course, Perl features that are not present in PCRE. The + nearest equivalent is the callout feature, as for example in this pat- + tern: + + a+(?C)(*FAIL) + + A match with the string "aaaa" always fails, but the callout is taken + before each backtrack happens (in this example, 10 times). + + Verbs that act after backtracking + + The following verbs do nothing when they are encountered. Matching con- + tinues with what follows, but if there is no subsequent match, a fail- + ure is forced. The verbs differ in exactly what kind of failure + occurs. + + (*COMMIT) + + This verb causes the whole match to fail outright if the rest of the + pattern does not match. Even if the pattern is unanchored, no further + attempts to find a match by advancing the start point take place. Once + (*COMMIT) has been passed, pcre_exec() is committed to finding a match + at the current starting point, or not at all. For example: + + a+(*COMMIT)b + + This matches "xxaab" but not "aacaab". It can be thought of as a kind + of dynamic anchor, or "I've started, so I must finish." + + (*PRUNE) + + This verb causes the match to fail at the current position if the rest + of the pattern does not match. If the pattern is unanchored, the normal + "bumpalong" advance to the next starting character then happens. Back- + tracking can occur as usual to the left of (*PRUNE), or when matching + to the right of (*PRUNE), but if there is no match to the right, back- + tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) + is just an alternative to an atomic group or possessive quantifier, but + there are some uses of (*PRUNE) that cannot be expressed in any other + way. + + (*SKIP) + + This verb is like (*PRUNE), except that if the pattern is unanchored, + the "bumpalong" advance is not to the next character, but to the posi- + tion in the subject where (*SKIP) was encountered. (*SKIP) signifies + that whatever text was matched leading up to it cannot be part of a + successful match. Consider: + + a+(*SKIP)b + + If the subject is "aaaac...", after the first match attempt fails + (starting at the first character in the string), the starting point + skips on to start the next attempt at "c". Note that a possessive quan- + tifer does not have the same effect in this example; although it would + suppress backtracking during the first match attempt, the second + attempt would start at the second character instead of skipping on to + "c". + + (*THEN) + + This verb causes a skip to the next alternation if the rest of the pat- + tern does not match. That is, it cancels pending backtracking, but only + within the current alternation. Its name comes from the observation + that it can be used for a pattern-based if-then-else block: + + ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... + + If the COND1 pattern matches, FOO is tried (and possibly further items + after the end of the group if FOO succeeds); on failure the matcher + skips to the second alternative and tries COND2, without backtracking + into COND1. If (*THEN) is used outside of any alternation, it acts + exactly like (*PRUNE). + + +SEE ALSO + + pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 19 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCRESYNTAX(3) PCRESYNTAX(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE REGULAR EXPRESSION SYNTAX SUMMARY + + The full syntax and semantics of the regular expressions that are sup- + ported by PCRE are described in the pcrepattern documentation. This + document contains just a quick-reference summary of the syntax. + + +QUOTING + + \x where x is non-alphanumeric is a literal x + \Q...\E treat enclosed characters as literal + + +CHARACTERS + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \ddd character with octal code ddd, or backreference + \xhh character with hex code hh + \x{hhh..} character with hex code hhh.. + + +CHARACTER TYPES + + . any character except newline; + in dotall mode, any character whatsoever + \C one byte, even in UTF-8 mode (best avoided) + \d a decimal digit + \D a character that is not a decimal digit + \h a horizontal whitespace character + \H a character that is not a horizontal whitespace character + \p{xx} a character with the xx property + \P{xx} a character without the xx property + \R a newline sequence + \s a whitespace character + \S a character that is not a whitespace character + \v a vertical whitespace character + \V a character that is not a vertical whitespace character + \w a "word" character + \W a "non-word" character + \X an extended Unicode sequence + + In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters. + + +GENERAL CATEGORY PROPERTY CODES FOR \p and \P + + C Other + Cc Control + Cf Format + Cn Unassigned + Co Private use + Cs Surrogate + + L Letter + Ll Lower case letter + Lm Modifier letter + Lo Other letter + Lt Title case letter + Lu Upper case letter + L& Ll, Lu, or Lt + + M Mark + Mc Spacing mark + Me Enclosing mark + Mn Non-spacing mark + + N Number + Nd Decimal number + Nl Letter number + No Other number + + P Punctuation + Pc Connector punctuation + Pd Dash punctuation + Pe Close punctuation + Pf Final punctuation + Pi Initial punctuation + Po Other punctuation + Ps Open punctuation + + S Symbol + Sc Currency symbol + Sk Modifier symbol + Sm Mathematical symbol + So Other symbol + + Z Separator + Zl Line separator + Zp Paragraph separator + Zs Space separator + + +SCRIPT NAMES FOR \p AND \P + + Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese, + Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform, + Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, + Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira- + gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, + Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko, + Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician, + Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, + Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi. + + +CHARACTER CLASSES + + [...] positive character class + [^...] negative character class + [x-y] range (can be used for hex characters) + [[:xxx:]] positive POSIX named set + [[:^xxx:]] negative POSIX named set + + alnum alphanumeric + alpha alphabetic + ascii 0-127 + blank space or tab + cntrl control character + digit decimal digit + graph printing, excluding space + lower lower case letter + print printing, including space + punct printing, excluding alphanumeric + space whitespace + upper upper case letter + word same as \w + xdigit hexadecimal digit + + In PCRE, POSIX character set names recognize only ASCII characters. You + can use \Q...\E inside a character class. + + +QUANTIFIERS + + ? 0 or 1, greedy + ?+ 0 or 1, possessive + ?? 0 or 1, lazy + * 0 or more, greedy + *+ 0 or more, possessive + *? 0 or more, lazy + + 1 or more, greedy + ++ 1 or more, possessive + +? 1 or more, lazy + {n} exactly n + {n,m} at least n, no more than m, greedy + {n,m}+ at least n, no more than m, possessive + {n,m}? at least n, no more than m, lazy + {n,} n or more, greedy + {n,}+ n or more, possessive + {n,}? n or more, lazy + + +ANCHORS AND SIMPLE ASSERTIONS + + \b word boundary + \B not a word boundary + ^ start of subject + also after internal newline in multiline mode + \A start of subject + $ end of subject + also before newline at end of subject + also before internal newline in multiline mode + \Z end of subject + also before newline at end of subject + \z end of subject + \G first matching position in subject + + +MATCH POINT RESET + + \K reset start of match + + +ALTERNATION + + expr|expr|expr... + + +CAPTURING + + (...) capturing group + (?...) named capturing group (Perl) + (?'name'...) named capturing group (Perl) + (?P...) named capturing group (Python) + (?:...) non-capturing group + (?|...) non-capturing group; reset group numbers for + capturing groups in each alternative + + +ATOMIC GROUPS + + (?>...) atomic, non-capturing group + + +COMMENT + + (?#....) comment (not nestable) + + +OPTION SETTING + + (?i) caseless + (?J) allow duplicate names + (?m) multiline + (?s) single line (dotall) + (?U) default ungreedy (lazy) + (?x) extended (ignore white space) + (?-...) unset option(s) + + +LOOKAHEAD AND LOOKBEHIND ASSERTIONS + + (?=...) positive look ahead + (?!...) negative look ahead + (?<=...) positive look behind + (? reference by name (Perl) + \k'name' reference by name (Perl) + \g{name} reference by name (Perl) + \k{name} reference by name (.NET) + (?P=name) reference by name (Python) + + +SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) + + (?R) recurse whole pattern + (?n) call subpattern by absolute number + (?+n) call subpattern by relative number + (?-n) call subpattern by relative number + (?&name) call subpattern by name (Perl) + (?P>name) call subpattern by name (Python) + \g call subpattern by name (Oniguruma) + \g'name' call subpattern by name (Oniguruma) + \g call subpattern by absolute number (Oniguruma) + \g'n' call subpattern by absolute number (Oniguruma) + \g<+n> call subpattern by relative number (PCRE extension) + \g'+n' call subpattern by relative number (PCRE extension) + \g<-n> call subpattern by relative number (PCRE extension) + \g'-n' call subpattern by relative number (PCRE extension) + + +CONDITIONAL PATTERNS + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + + (?(n)... absolute reference condition + (?(+n)... relative reference condition + (?(-n)... relative reference condition + (?()... named reference condition (Perl) + (?('name')... named reference condition (Perl) + (?(name)... named reference condition (PCRE) + (?(R)... overall recursion condition + (?(Rn)... specific group recursion condition + (?(R&name)... specific recursion condition + (?(DEFINE)... define subpattern for reference + (?(assert)... assertion condition + + +BACKTRACKING CONTROL + + The following act immediately they are reached: + + (*ACCEPT) force successful match + (*FAIL) force backtrack; synonym (*F) + + The following act only when a subsequent match failure causes a back- + track to reach them. They all force a match failure, but they differ in + what happens afterwards. Those that advance the start-of-match point do + so only if the pattern is not anchored. + + (*COMMIT) overall failure, no advance of starting point + (*PRUNE) advance to next starting character + (*SKIP) advance start to current matching position + (*THEN) local failure, backtrack to next alternation + + +NEWLINE CONVENTIONS + + These are recognized only at the very start of the pattern or after a + (*BSR_...) option. + + (*CR) + (*LF) + (*CRLF) + (*ANYCRLF) + (*ANY) + + +WHAT \R MATCHES + + These are recognized only at the very start of the pattern or after a + (*...) option that sets the newline convention. + + (*BSR_ANYCRLF) + (*BSR_UNICODE) + + +CALLOUTS + + (?C) callout + (?Cn) callout with data n + + +SEE ALSO + + pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 09 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREPARTIAL(3) PCREPARTIAL(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PARTIAL MATCHING IN PCRE + + In normal use of PCRE, if the subject string that is passed to + pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too + short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. + There are circumstances where it might be helpful to distinguish this + case from other cases in which there is no match. + + Consider, for example, an application where a human is required to type + in data for a field with specific formatting requirements. An example + might be a date in the form ddmmmyy, defined by this pattern: + + ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ + + If the application sees the user's keystrokes one by one, and can check + that what has been typed so far is potentially valid, it is able to + raise an error as soon as a mistake is made, possibly beeping and not + reflecting the character that has been typed. This immediate feedback + is likely to be a better user interface than a check that is delayed + until the entire string has been entered. + + PCRE supports the concept of partial matching by means of the PCRE_PAR- + TIAL option, which can be set when calling pcre_exec() or + pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code + PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time + during the matching process the last part of the subject string matched + part of the pattern. Unfortunately, for non-anchored matching, it is + not possible to obtain the position of the start of the partial match. + No captured data is set when PCRE_ERROR_PARTIAL is returned. + + When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code + PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of + the subject is reached, there have been no complete matches, but there + is still at least one matching possibility. The portion of the string + that provided the partial match is set as the first matching string. + + Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers + the last literal byte in a pattern, and abandons matching immediately + if such a byte is not present in the subject string. This optimization + cannot be used for a subject string that might match only partially. + + +RESTRICTED PATTERNS FOR PCRE_PARTIAL + + Because of the way certain internal optimizations are implemented in + the pcre_exec() function, the PCRE_PARTIAL option cannot be used with + all patterns. These restrictions do not apply when pcre_dfa_exec() is + used. For pcre_exec(), repeated single characters such as + + a{2,4} + + and repeated single metasequences such as + + \d+ + + are not permitted if the maximum number of occurrences is greater than + one. Optional items such as \d? (where the maximum is one) are permit- + ted. Quantifiers with any values are permitted after parentheses, so + the invalid examples above can be coded thus: + + (a){2,4} + (\d)+ + + These constructions run more slowly, but for the kinds of application + that are envisaged for this facility, this is not felt to be a major + restriction. + + If PCRE_PARTIAL is set for a pattern that does not conform to the + restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL + (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to + find out if a compiled pattern can be used for partial matching. + + +EXAMPLE OF PARTIAL MATCHING USING PCRETEST + + If the escape sequence \P is present in a pcretest data line, the + PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that + uses the date example quoted above: + + re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 25jun04\P + 0: 25jun04 + 1: jun + data> 25dec3\P + Partial match + data> 3ju\P + Partial match + data> 3juj\P + No match + data> j\P + No match + + The first data string is matched completely, so pcretest shows the + matched substrings. The remaining four strings do not match the com- + plete pattern, but the first two are partial matches. The same test, + using pcre_dfa_exec() matching (by means of the \D escape sequence), + produces the following output: + + re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 25jun04\P\D + 0: 25jun04 + data> 23dec3\P\D + Partial match: 23dec3 + data> 3ju\P\D + Partial match: 3ju + data> 3juj\P\D + No match + data> j\P\D + No match + + Notice that in this case the portion of the string that was matched is + made available. + + +MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() + + When a partial match has been found using pcre_dfa_exec(), it is possi- + ble to continue the match by providing additional subject data and + calling pcre_dfa_exec() again with the same compiled regular expres- + sion, this time setting the PCRE_DFA_RESTART option. You must also pass + the same working space as before, because this is where details of the + previous partial match are stored. Here is an example using pcretest, + using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and + \D are as above): + + re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 23ja\P\D + Partial match: 23ja + data> n05\R\D + 0: n05 + + The first call has "23ja" as the subject, and requests partial match- + ing; the second call has "n05" as the subject for the continued + (restarted) match. Notice that when the match is complete, only the + last part is shown; PCRE does not retain the previously partially- + matched string. It is up to the calling program to do that if it needs + to. + + You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial + matching over multiple segments. This facility can be used to pass very + long subject strings to pcre_dfa_exec(). However, some care is needed + for certain types of pattern. + + 1. If the pattern contains tests for the beginning or end of a line, + you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri- + ate, when the subject string for any call does not contain the begin- + ning or end of a line. + + 2. If the pattern contains backward assertions (including \b or \B), + you need to arrange for some overlap in the subject strings to allow + for this. For example, you could pass the subject in chunks that are + 500 bytes long, but in a buffer of 700 bytes, with the starting offset + set to 200 and the previous 200 bytes at the start of the buffer. + + 3. Matching a subject string that is split into multiple segments does + not always produce exactly the same result as matching over one single + long string. The difference arises when there are multiple matching + possibilities, because a partial match result is given only when there + are no completed matches in a call to pcre_dfa_exec(). This means that + as soon as the shortest match has been found, continuation to a new + subject segment is no longer possible. Consider this pcretest example: + + re> /dog(sbody)?/ + data> do\P\D + Partial match: do + data> gsb\R\P\D + 0: g + data> dogsbody\D + 0: dogsbody + 1: dog + + The pattern matches the words "dog" or "dogsbody". When the subject is + presented in several parts ("do" and "gsb" being the first two) the + match stops when "dog" has been found, and it is not possible to con- + tinue. On the other hand, if "dogsbody" is presented as a single + string, both matches are found. + + Because of this phenomenon, it does not usually make sense to end a + pattern that is going to be matched in this way with a variable repeat. + + 4. Patterns that contain alternatives at the top level which do not all + start with the same pattern item may not work as expected. For example, + consider this pattern: + + 1234|3789 + + If the first part of the subject is "ABC123", a partial match of the + first alternative is found at offset 3. There is no partial match for + the second alternative, because such a match does not start at the same + point in the subject string. Attempting to continue with the string + "789" does not yield a match because only those alternatives that match + at one point in the subject are remembered. The problem arises because + the start of the second alternative matches within the first alterna- + tive. There is no problem with anchored patterns or patterns such as: + + 1234|ABCD + + where no string can be a partial match for both alternatives. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 04 June 2007 + Copyright (c) 1997-2007 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREPRECOMPILE(3) PCREPRECOMPILE(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +SAVING AND RE-USING PRECOMPILED PCRE PATTERNS + + If you are running an application that uses a large number of regular + expression patterns, it may be useful to store them in a precompiled + form instead of having to compile them every time the application is + run. If you are not using any private character tables (see the + pcre_maketables() documentation), this is relatively straightforward. + If you are using private tables, it is a little bit more complicated. + + If you save compiled patterns to a file, you can copy them to a differ- + ent host and run them there. This works even if the new host has the + opposite endianness to the one on which the patterns were compiled. + There may be a small performance penalty, but it should be insignifi- + cant. However, compiling regular expressions with one version of PCRE + for use with a different version is not guaranteed to work and may + cause crashes. + + +SAVING A COMPILED PATTERN + The value returned by pcre_compile() points to a single block of memory + that holds the compiled pattern and associated data. You can find the + length of this block in bytes by calling pcre_fullinfo() with an argu- + ment of PCRE_INFO_SIZE. You can then save the data in any appropriate + manner. Here is sample code that compiles a pattern and writes it to a + file. It assumes that the variable fd refers to a file that is open for + output: + + int erroroffset, rc, size; + char *error; + pcre *re; + + re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); + if (re == NULL) { ... handle errors ... } + rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); + if (rc < 0) { ... handle errors ... } + rc = fwrite(re, 1, size, fd); + if (rc != size) { ... handle errors ... } + + In this example, the bytes that comprise the compiled pattern are + copied exactly. Note that this is binary data that may contain any of + the 256 possible byte values. On systems that make a distinction + between binary and non-binary data, be sure that the file is opened for + binary output. + + If you want to write more than one pattern to a file, you will have to + devise a way of separating them. For binary data, preceding each pat- + tern with its length is probably the most straightforward approach. + Another possibility is to write out the data in hexadecimal instead of + binary, one pattern to a line. + + Saving compiled patterns in a file is only one possible way of storing + them for later use. They could equally well be saved in a database, or + in the memory of some daemon process that passes them via sockets to + the processes that want them. + + If the pattern has been studied, it is also possible to save the study + data in a similar way to the compiled pattern itself. When studying + generates additional information, pcre_study() returns a pointer to a + pcre_extra data block. Its format is defined in the section on matching + a pattern in the pcreapi documentation. The study_data field points to + the binary study data, and this is what you must save (not the + pcre_extra block itself). The length of the study data can be obtained + by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. + Remember to check that pcre_study() did return a non-NULL value before + trying to save the study data. + + +RE-USING A PRECOMPILED PATTERN + + Re-using a precompiled pattern is straightforward. Having reloaded it + into main memory, you pass its pointer to pcre_exec() or + pcre_dfa_exec() in the usual way. This should work even on another + host, and even if that host has the opposite endianness to the one + where the pattern was compiled. + + However, if you passed a pointer to custom character tables when the + pattern was compiled (the tableptr argument of pcre_compile()), you + must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(), + because the value saved with the compiled pattern will obviously be + nonsense. A field in a pcre_extra() block is used to pass this data, as + described in the section on matching a pattern in the pcreapi documen- + tation. + + If you did not provide custom character tables when the pattern was + compiled, the pointer in the compiled pattern is NULL, which causes + pcre_exec() to use PCRE's internal tables. Thus, you do not need to + take any special action at run time in this case. + + If you saved study data with the compiled pattern, you need to create + your own pcre_extra data block and set the study_data field to point to + the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA + bit in the flags field to indicate that study data is present. Then + pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the + usual way. + + +COMPATIBILITY WITH DIFFERENT PCRE RELEASES + + In general, it is safest to recompile all saved patterns when you + update to a new PCRE release, though not all updates actually require + this. Recompiling is definitely needed for release 7.2. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 13 June 2007 + Copyright (c) 1997-2007 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREPERFORM(3) PCREPERFORM(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE PERFORMANCE + + Two aspects of performance are discussed below: memory usage and pro- + cessing time. The way you express your pattern as a regular expression + can affect both of them. + + +MEMORY USAGE + + Patterns are compiled by PCRE into a reasonably efficient byte code, so + that most simple patterns do not use much memory. However, there is one + case where memory usage can be unexpectedly large. When a parenthesized + subpattern has a quantifier with a minimum greater than 1 and/or a lim- + ited maximum, the whole subpattern is repeated in the compiled code. + For example, the pattern + + (abc|def){2,4} + + is compiled as if it were + + (abc|def)(abc|def)((abc|def)(abc|def)?)? + + (Technical aside: It is done this way so that backtrack points within + each of the repetitions can be independently maintained.) + + For regular expressions whose quantifiers use only small numbers, this + is not usually a problem. However, if the numbers are large, and par- + ticularly if such repetitions are nested, the memory usage can become + an embarrassment. For example, the very simple pattern + + ((ab){1,1000}c){1,3} + + uses 51K bytes when compiled. When PCRE is compiled with its default + internal pointer size of two bytes, the size limit on a compiled pat- + tern is 64K, and this is reached with the above pattern if the outer + repetition is increased from 3 to 4. PCRE can be compiled to use larger + internal pointers and thus handle larger compiled patterns, but it is + better to try to rewrite your pattern to use less memory if you can. + + One way of reducing the memory usage for such patterns is to make use + of PCRE's "subroutine" facility. Re-writing the above pattern as + + ((ab)(?2){0,999}c)(?1){0,2} + + reduces the memory requirements to 18K, and indeed it remains under 20K + even with the outer repetition increased to 100. However, this pattern + is not exactly equivalent, because the "subroutine" calls are treated + as atomic groups into which there can be no backtracking if there is a + subsequent matching failure. Therefore, PCRE cannot do this kind of + rewriting automatically. Furthermore, there is a noticeable loss of + speed when executing the modified pattern. Nevertheless, if the atomic + grouping is not a problem and the loss of speed is acceptable, this + kind of rewriting will allow you to process patterns that PCRE cannot + otherwise handle. + + +PROCESSING TIME + + Certain items in regular expression patterns are processed more effi- + ciently than others. It is more efficient to use a character class like + [aeiou] than a set of single-character alternatives such as + (a|e|i|o|u). In general, the simplest construction that provides the + required behaviour is usually the most efficient. Jeffrey Friedl's book + contains a lot of useful general discussion about optimizing regular + expressions for efficient performance. This document contains a few + observations about PCRE. + + Using Unicode character properties (the \p, \P, and \X escapes) is + slow, because PCRE has to scan a structure that contains data for over + fifteen thousand characters whenever it needs a character's property. + If you can find an alternative pattern that does not use character + properties, it will probably be faster. + + When a pattern begins with .* not in parentheses, or in parentheses + that are not the subject of a backreference, and the PCRE_DOTALL option + is set, the pattern is implicitly anchored by PCRE, since it can match + only at the start of a subject string. However, if PCRE_DOTALL is not + set, PCRE cannot make this optimization, because the . metacharacter + does not then match a newline, and if the subject string contains new- + lines, the pattern may match from the character immediately following + one of them instead of from the very start. For example, the pattern + + .*second + + matches the subject "first\nand second" (where \n stands for a newline + character), with the match starting at the seventh character. In order + to do this, PCRE has to retry the match starting after every newline in + the subject. + + If you are using such a pattern with subject strings that do not con- + tain newlines, the best performance is obtained by setting PCRE_DOTALL, + or starting the pattern with ^.* or ^.*? to indicate explicit anchor- + ing. That saves PCRE from having to scan along the subject looking for + a newline to restart at. + + Beware of patterns that contain nested indefinite repeats. These can + take a long time to run when applied to a string that does not match. + Consider the pattern fragment + + ^(a+)* + + This can match "aaaa" in 16 different ways, and this number increases + very rapidly as the string gets longer. (The * repeat can match 0, 1, + 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + + repeats can match different numbers of times.) When the remainder of + the pattern is such that the entire match is going to fail, PCRE has in + principle to try every possible variation, and this can take an + extremely long time, even for relatively short strings. + + An optimization catches some of the more simple cases such as + + (a+)*b + + where a literal character follows. Before embarking on the standard + matching procedure, PCRE checks that there is a "b" later in the sub- + ject string, and if there is not, it fails the match immediately. How- + ever, when there is no following literal this optimization cannot be + used. You can see the difference by comparing the behaviour of + + (a+)*\d + + with the pattern above. The former gives a failure almost instantly + when applied to a whole line of "a" characters, whereas the latter + takes an appreciable time with strings longer than about 20 characters. + + In many cases, the solution to this kind of performance issue is to use + an atomic group or a possessive quantifier. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 06 March 2007 + Copyright (c) 1997-2007 University of Cambridge. +------------------------------------------------------------------------------ + + +PCREPOSIX(3) PCREPOSIX(3) + + +NAME + PCRE - Perl-compatible regular expressions. + + +SYNOPSIS OF POSIX API + + #include + + int regcomp(regex_t *preg, const char *pattern, + int cflags); + + int regexec(regex_t *preg, const char *string, + size_t nmatch, regmatch_t pmatch[], int eflags); + + size_t regerror(int errcode, const regex_t *preg, + char *errbuf, size_t errbuf_size); + + void regfree(regex_t *preg); + + +DESCRIPTION + + This set of functions provides a POSIX-style API to the PCRE regular + expression package. See the pcreapi documentation for a description of + PCRE's native API, which contains much additional functionality. + + The functions described here are just wrapper functions that ultimately + call the PCRE native API. Their prototypes are defined in the + pcreposix.h header file, and on Unix systems the library itself is + called pcreposix.a, so can be accessed by adding -lpcreposix to the + command for linking an application that uses them. Because the POSIX + functions call the native ones, it is also necessary to add -lpcre. + + I have implemented only those option bits that can be reasonably mapped + to PCRE native options. In addition, the option REG_EXTENDED is defined + with the value zero. This has no effect, but since programs that are + written to the POSIX interface often use it, this makes it easier to + slot in PCRE as a replacement library. Other POSIX options are not even + defined. + + When PCRE is called via these functions, it is only the API that is + POSIX-like in style. The syntax and semantics of the regular expres- + sions themselves are still those of Perl, subject to the setting of + various PCRE options, as described below. "POSIX-like in style" means + that the API approximates to the POSIX definition; it is not fully + POSIX-compatible, and in multi-byte encoding domains it is probably + even less compatible. + + The header for these functions is supplied as pcreposix.h to avoid any + potential clash with other POSIX libraries. It can, of course, be + renamed or aliased as regex.h, which is the "correct" name. It provides + two structure types, regex_t for compiled internal forms, and reg- + match_t for returning captured substrings. It also defines some con- + stants whose names start with "REG_"; these are used for setting + options and identifying error codes. + + +COMPILING A PATTERN + + The function regcomp() is called to compile a pattern into an internal + form. The pattern is a C string terminated by a binary zero, and is + passed in the argument pattern. The preg argument is a pointer to a + regex_t structure that is used as a base for storing information about + the compiled regular expression. + + The argument cflags is either zero, or contains one or more of the bits + defined by the following macros: + + REG_DOTALL + + The PCRE_DOTALL option is set when the regular expression is passed for + compilation to the native function. Note that REG_DOTALL is not part of + the POSIX standard. + + REG_ICASE + + The PCRE_CASELESS option is set when the regular expression is passed + for compilation to the native function. + + REG_NEWLINE + + The PCRE_MULTILINE option is set when the regular expression is passed + for compilation to the native function. Note that this does not mimic + the defined POSIX behaviour for REG_NEWLINE (see the following sec- + tion). + + REG_NOSUB + + The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is + passed for compilation to the native function. In addition, when a pat- + tern that is compiled with this flag is passed to regexec() for match- + ing, the nmatch and pmatch arguments are ignored, and no captured + strings are returned. + + REG_UTF8 + + The PCRE_UTF8 option is set when the regular expression is passed for + compilation to the native function. This causes the pattern itself and + all data strings used for matching it to be treated as UTF-8 strings. + Note that REG_UTF8 is not part of the POSIX standard. + + In the absence of these flags, no options are passed to the native + function. This means the the regex is compiled with PCRE default + semantics. In particular, the way it handles newline characters in the + subject string is the Perl way, not the POSIX way. Note that setting + PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. + It does not affect the way newlines are matched by . (they aren't) or + by a negative class such as [^a] (they are). + + The yield of regcomp() is zero on success, and non-zero otherwise. The + preg structure is filled in on success, and one member of the structure + is public: re_nsub contains the number of capturing subpatterns in the + regular expression. Various error codes are defined in the header file. + + +MATCHING NEWLINE CHARACTERS + + This area is not simple, because POSIX and Perl take different views of + things. It is not possible to get PCRE to obey POSIX semantics, but + then PCRE was never intended to be a POSIX engine. The following table + lists the different possibilities for matching newline characters in + PCRE: + + Default Change with + + . matches newline no PCRE_DOTALL + newline matches [^a] yes not changeable + $ matches \n at end yes PCRE_DOLLARENDONLY + $ matches \n in middle no PCRE_MULTILINE + ^ matches \n in middle no PCRE_MULTILINE + + This is the equivalent table for POSIX: + + Default Change with + + . matches newline yes REG_NEWLINE + newline matches [^a] yes REG_NEWLINE + $ matches \n at end no REG_NEWLINE + $ matches \n in middle no REG_NEWLINE + ^ matches \n in middle no REG_NEWLINE + + PCRE's behaviour is the same as Perl's, except that there is no equiva- + lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is + no way to stop newline from matching [^a]. + + The default POSIX newline handling can be obtained by setting + PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE + behave exactly as for the REG_NEWLINE action. + + +MATCHING A PATTERN + + The function regexec() is called to match a compiled pattern preg + against a given string, which is by default terminated by a zero byte + (but see REG_STARTEND below), subject to the options in eflags. These + can be: + + REG_NOTBOL + + The PCRE_NOTBOL option is set when calling the underlying PCRE matching + function. + + REG_NOTEOL + + The PCRE_NOTEOL option is set when calling the underlying PCRE matching + function. + + REG_STARTEND + + The string is considered to start at string + pmatch[0].rm_so and to + have a terminating NUL located at string + pmatch[0].rm_eo (there need + not actually be a NUL at that location), regardless of the value of + nmatch. This is a BSD extension, compatible with but not specified by + IEEE Standard 1003.2 (POSIX.2), and should be used with caution in + software intended to be portable to other systems. Note that a non-zero + rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location + of the string, not how it is matched. + + If the pattern was compiled with the REG_NOSUB flag, no data about any + matched strings is returned. The nmatch and pmatch arguments of + regexec() are ignored. + + Otherwise,the portion of the string that was matched, and also any cap- + tured substrings, are returned via the pmatch argument, which points to + an array of nmatch structures of type regmatch_t, containing the mem- + bers rm_so and rm_eo. These contain the offset to the first character + of each substring and the offset to the first character after the end + of each substring, respectively. The 0th element of the vector relates + to the entire portion of string that was matched; subsequent elements + relate to the capturing subpatterns of the regular expression. Unused + entries in the array have both structure members set to -1. + + A successful match yields a zero return; various error codes are + defined in the header file, of which REG_NOMATCH is the "expected" + failure code. + + +ERROR MESSAGES + + The regerror() function maps a non-zero errorcode from either regcomp() + or regexec() to a printable message. If preg is not NULL, the error + should have arisen from the use of that structure. A message terminated + by a binary zero is placed in errbuf. The length of the message, + including the zero, is limited to errbuf_size. The yield of the func- + tion is the size of buffer needed to hold the whole message. + + +MEMORY USAGE + + Compiling a regular expression causes memory to be allocated and asso- + ciated with the preg structure. The function regfree() frees all such + memory, after which preg may no longer be used as a compiled expres- + sion. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 05 April 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ + + +PCRECPP(3) PCRECPP(3) + + +NAME + PCRE - Perl-compatible regular expressions. + + +SYNOPSIS OF C++ WRAPPER + + #include + + +DESCRIPTION + + The C++ wrapper for PCRE was provided by Google Inc. Some additional + functionality was added by Giuseppe Maxia. This brief man page was con- + structed from the notes in the pcrecpp.h file, which should be con- + sulted for further details. + + +MATCHING INTERFACE + + The "FullMatch" operation checks that supplied text matches a supplied + pattern exactly. If pointer arguments are supplied, it copies matched + sub-strings that match sub-patterns into them. + + Example: successful match + pcrecpp::RE re("h.*o"); + re.FullMatch("hello"); + + Example: unsuccessful match (requires full match): + pcrecpp::RE re("e"); + !re.FullMatch("hello"); + + Example: creating a temporary RE object: + pcrecpp::RE("h.*o").FullMatch("hello"); + + You can pass in a "const char*" or a "string" for "text". The examples + below tend to use a const char*. You can, as in the different examples + above, store the RE object explicitly in a variable or use a temporary + RE object. The examples below use one mode or the other arbitrarily. + Either could correctly be used for any of these examples. + + You must supply extra pointer arguments to extract matched subpieces. + + Example: extracts "ruby" into "s" and 1234 into "i" + int i; + string s; + pcrecpp::RE re("(\\w+):(\\d+)"); + re.FullMatch("ruby:1234", &s, &i); + + Example: does not try to extract any extra sub-patterns + re.FullMatch("ruby:1234", &s); + + Example: does not try to extract into NULL + re.FullMatch("ruby:1234", NULL, &i); + + Example: integer overflow causes failure + !re.FullMatch("ruby:1234567891234", NULL, &i); + + Example: fails because there aren't enough sub-patterns: + !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); + + Example: fails because string cannot be stored in integer + !pcrecpp::RE("(.*)").FullMatch("ruby", &i); + + The provided pointer arguments can be pointers to any scalar numeric + type, or one of: + + string (matched piece is copied to string) + StringPiece (StringPiece is mutated to point to matched piece) + T (where "bool T::ParseFrom(const char*, int)" exists) + NULL (the corresponding matched sub-pattern is not copied) + + The function returns true iff all of the following conditions are sat- + isfied: + + a. "text" matches "pattern" exactly; + + b. The number of matched sub-patterns is >= number of supplied + pointers; + + c. The "i"th argument has a suitable type for holding the + string captured as the "i"th sub-pattern. If you pass in + void * NULL for the "i"th argument, or a non-void * NULL + of the correct type, or pass fewer arguments than the + number of sub-patterns, "i"th captured sub-pattern is + ignored. + + CAVEAT: An optional sub-pattern that does not exist in the matched + string is assigned the empty string. Therefore, the following will + return false (because the empty string is not a valid number): + + int number; + pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); + + The matching interface supports at most 16 arguments per call. If you + need more, consider using the more general interface + pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. + + +QUOTING METACHARACTERS + + You can use the "QuoteMeta" operation to insert backslashes before all + potentially meaningful characters in a string. The returned string, + used as a regular expression, will exactly match the original string. + + Example: + string quoted = RE::QuoteMeta(unquoted); + + Note that it's legal to escape a character even if it has no special + meaning in a regular expression -- so this function does that. (This + also makes it identical to the perl function of the same name; see + "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes + "1\.5\-2\.0\?". + + +PARTIAL MATCHES + + You can use the "PartialMatch" operation when you want the pattern to + match any substring of the text. + + Example: simple search for a string: + pcrecpp::RE("ell").PartialMatch("hello"); + + Example: find first number in a string: + int number; + pcrecpp::RE re("(\\d+)"); + re.PartialMatch("x*100 + 20", &number); + assert(number == 100); + + +UTF-8 AND THE MATCHING INTERFACE + + By default, pattern and text are plain text, one byte per character. + The UTF8 flag, passed to the constructor, causes both pattern and + string to be treated as UTF-8 text, still a byte stream but potentially + multiple bytes per character. In practice, the text is likelier to be + UTF-8 than the pattern, but the match returned may depend on the UTF8 + flag, so always use it when matching UTF8 text. For example, "." will + match one byte normally but with UTF8 set may match up to three bytes + of a multi-byte character. + + Example: + pcrecpp::RE_Options options; + options.set_utf8(); + pcrecpp::RE re(utf8_pattern, options); + re.FullMatch(utf8_string); + + Example: using the convenience function UTF8(): + pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); + re.FullMatch(utf8_string); + + NOTE: The UTF8 flag is ignored if pcre was not configured with the + --enable-utf8 flag. + + +PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE + + PCRE defines some modifiers to change the behavior of the regular + expression engine. The C++ wrapper defines an auxiliary class, + RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- + rently, the following modifiers are supported: + + modifier description Perl corresponding + + PCRE_CASELESS case insensitive match /i + PCRE_MULTILINE multiple lines match /m + PCRE_DOTALL dot matches newlines /s + PCRE_DOLLAR_ENDONLY $ matches only at end N/A + PCRE_EXTRA strict escape parsing N/A + PCRE_EXTENDED ignore whitespaces /x + PCRE_UTF8 handles UTF8 chars built-in + PCRE_UNGREEDY reverses * and *? N/A + PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) + + (*) Both Perl and PCRE allow non capturing parentheses by means of the + "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- + ture, while (ab|cd) does. + + For a full account on how each modifier works, please check the PCRE + API reference page. + + For each modifier, there are two member functions whose name is made + out of the modifier in lowercase, without the "PCRE_" prefix. For + instance, PCRE_CASELESS is handled by + + bool caseless() + + which returns true if the modifier is set, and + + RE_Options & set_caseless(bool) + + which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can + be accessed through the set_match_limit() and match_limit() member + functions. Setting match_limit to a non-zero value will limit the exe- + cution of pcre to keep it from doing bad things like blowing the stack + or taking an eternity to return a result. A value of 5000 is good + enough to stop stack blowup in a 2MB thread stack. Setting match_limit + to zero disables match limiting. Alternatively, you can call + match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to + limit how much PCRE recurses. match_limit() limits the number of + matches PCRE does; match_limit_recursion() limits the depth of internal + recursion, and therefore the amount of stack that is used. + + Normally, to pass one or more modifiers to a RE class, you declare a + RE_Options object, set the appropriate options, and pass this object to + a RE constructor. Example: + + RE_options opt; + opt.set_caseless(true); + if (RE("HELLO", opt).PartialMatch("hello world")) ... + + RE_options has two constructors. The default constructor takes no argu- + ments and creates a set of flags that are off by default. The optional + parameter option_flags is to facilitate transfer of legacy code from C + programs. This lets you do + + RE(pattern, + RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); + + However, new code is better off doing + + RE(pattern, + RE_Options().set_caseless(true).set_multiline(true)) + .PartialMatch(str); + + If you are going to pass one of the most used modifiers, there are some + convenience functions that return a RE_Options class with the appropri- + ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), + and EXTENDED(). + + If you need to set several options at once, and you don't want to go + through the pains of declaring a RE_Options object and setting several + options, there is a parallel method that give you such ability on the + fly. You can concatenate several set_xxxxx() member functions, since + each of them returns a reference to its class object. For example, to + pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one + statement, you may write: + + RE(" ^ xyz \\s+ .* blah$", + RE_Options() + .set_caseless(true) + .set_extended(true) + .set_multiline(true)).PartialMatch(sometext); + + +SCANNING TEXT INCREMENTALLY + + The "Consume" operation may be useful if you want to repeatedly match + regular expressions at the front of a string and skip over them as they + match. This requires use of the "StringPiece" type, which represents a + sub-range of a real string. Like RE, StringPiece is defined in the + pcrecpp namespace. + + Example: read lines of the form "var = value" from a string. + string contents = ...; // Fill string somehow + pcrecpp::StringPiece input(contents); // Wrap in a StringPiece + + string var; + int value; + pcrecpp::RE re("(\\w+) = (\\d+)\n"); + while (re.Consume(&input, &var, &value)) { + ...; + } + + Each successful call to "Consume" will set "var/value", and also + advance "input" so it points past the matched text. + + The "FindAndConsume" operation is similar to "Consume" but does not + anchor your match at the beginning of the string. For example, you + could extract all words from a string by repeatedly calling + + pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) + + +PARSING HEX/OCTAL/C-RADIX NUMBERS + + By default, if you pass a pointer to a numeric value, the corresponding + text is interpreted as a base-10 number. You can instead wrap the + pointer with a call to one of the operators Hex(), Octal(), or CRadix() + to interpret the text in another base. The CRadix operator interprets + C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to + base-10. + + Example: + int a, b, c, d; + pcrecpp::RE re("(.*) (.*) (.*) (.*)"); + re.FullMatch("100 40 0100 0x40", + pcrecpp::Octal(&a), pcrecpp::Hex(&b), + pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); + + will leave 64 in a, b, c, and d. + + +REPLACING PARTS OF STRINGS + + You can replace the first match of "pattern" in "str" with "rewrite". + Within "rewrite", backslash-escaped digits (\1 to \9) can be used to + insert text matching corresponding parenthesized group from the pat- + tern. \0 in "rewrite" refers to the entire matching text. For example: + + string s = "yabba dabba doo"; + pcrecpp::RE("b+").Replace("d", &s); + + will leave "s" containing "yada dabba doo". The result is true if the + pattern matches and a replacement occurs, false otherwise. + + GlobalReplace is like Replace except that it replaces all occurrences + of the pattern in the string with the rewrite. Replacements are not + subject to re-matching. For example: + + string s = "yabba dabba doo"; + pcrecpp::RE("b+").GlobalReplace("d", &s); + + will leave "s" containing "yada dada doo". It returns the number of + replacements made. + + Extract is like Replace, except that if the pattern matches, "rewrite" + is copied into "out" (an additional argument) with substitutions. The + non-matching portions of "text" are ignored. Returns true iff a match + occurred and the extraction happened successfully; if no match occurs, + the string is left unaffected. + + +AUTHOR + + The C++ wrapper was contributed by Google Inc. + Copyright (c) 2007 Google Inc. + + +REVISION + + Last updated: 12 November 2007 +------------------------------------------------------------------------------ + + +PCRESAMPLE(3) PCRESAMPLE(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE SAMPLE PROGRAM + + A simple, complete demonstration program, to get you started with using + PCRE, is supplied in the file pcredemo.c in the PCRE distribution. + + The program compiles the regular expression that is its first argument, + and matches it against the subject string in its second argument. No + PCRE options are set, and default character tables are used. If match- + ing succeeds, the program outputs the portion of the subject that + matched, together with the contents of any captured substrings. + + If the -g option is given on the command line, the program then goes on + to check for further matches of the same regular expression in the same + subject string. The logic is a little bit tricky because of the possi- + bility of matching an empty string. Comments in the code explain what + is going on. + + If PCRE is installed in the standard include and library directories + for your system, you should be able to compile the demonstration pro- + gram using this command: + + gcc -o pcredemo pcredemo.c -lpcre + + If PCRE is installed elsewhere, you may need to add additional options + to the command line. For example, on a Unix-like system that has PCRE + installed in /usr/local, you can compile the demonstration program + using a command like this: + + gcc -o pcredemo -I/usr/local/include pcredemo.c \ + -L/usr/local/lib -lpcre + + Once you have compiled the demonstration program, you can run simple + tests like this: + + ./pcredemo 'cat|dog' 'the cat sat on the mat' + ./pcredemo -g 'cat|dog' 'the dog sat on the cat' + + Note that there is a much more comprehensive test program, called + pcretest, which supports many more facilities for testing regular + expressions and the PCRE library. The pcredemo program is provided as a + simple coding example. + + On some operating systems (e.g. Solaris), when PCRE is not installed in + the standard library directory, you may get an error like this when you + try to run pcredemo: + + ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or + directory + + This is caused by the way shared library support works on those sys- + tems. You need to add + + -R/usr/local/lib + + (for example) to the compile command to get round this problem. + + +AUTHOR + + Philip Hazel + University Computing Service + Cambridge CB2 3QH, England. + + +REVISION + + Last updated: 23 January 2008 + Copyright (c) 1997-2008 University of Cambridge. +------------------------------------------------------------------------------ +PCRESTACK(3) PCRESTACK(3) + + +NAME + PCRE - Perl-compatible regular expressions + + +PCRE DISCUSSION OF STACK USAGE + + When you call pcre_exec(), it makes use of an internal function called + match(). This calls itself recursively at branch points in the pattern, + in order to remember the state of the match so that it can back up and + try a different alternative if the first one fails. As matching pro- + ceeds deeper and deeper into the tree of possibilities, the recursion + depth increases. + + Not all calls of match() increase the recursion depth; for an item such + as a* it may be called several times at the same level, after matching + different numbers of a's. Furthermore, in a number of cases where the + result of the recursive call would immediately be passed back as the + result of the current call (a "tail recursion"), the function is just + restarted instead. + + The pcre_dfa_exec() function operates in an entirely different way, and + hardly uses recursion at all. The limit on its complexity is the amount + of workspace it is given. The comments that follow do NOT apply to + pcre_dfa_exec(); they are relevant only for pcre_exec(). + + You can set limits on the number of times that match() is called, both + in total and recursively. If the limit is exceeded, an error occurs. + For details, see the section on extra data for pcre_exec() in the + pcreapi documentation. + + Each time that match() is actually called recursively, it uses memory + from the process stack. For certain kinds of pattern and data, very + large amounts of stack may be needed, despite the recognition of "tail + recursion". You can often reduce the amount of recursion, and there- + fore the amount of stack used, by modifying the pattern that is being + matched. Consider, for example, this pattern: + + ([^<]|<(?!inet))+ + + It matches from wherever it starts until it encounters "