1 .TH PCREPRECOMPILE 3

     2 .SH NAME

     3 PCRE - Perl-compatible regular expressions

     4 .SH "SAVING AND RE-USING PRECOMPILED PCRE PATTERNS"

     5 .rs

     6 .sp

     7 If you are running an application that uses a large number of regular

     8 expression patterns, it may be useful to store them in a precompiled form

     9 instead of having to compile them every time the application is run.

    10 If you are not using any private character tables (see the

    11 .\" HREF

    12 \fBpcre_maketables()\fP

    13 .\"

    14 documentation), this is relatively straightforward. If you are using private

    15 tables, it is a little bit more complicated.

    16 .P

    17 If you save compiled patterns to a file, you can copy them to a different host

    18 and run them there. This works even if the new host has the opposite endianness

    19 to the one on which the patterns were compiled. There may be a small

    20 performance penalty, but it should be insignificant. However, compiling regular

    21 expressions with one version of PCRE for use with a different version is not

    22 guaranteed to work and may cause crashes.

    23 .

    24 .

    25 .SH "SAVING A COMPILED PATTERN"

    26 .rs

    27 .sh

    28 The value returned by \fBpcre_compile()\fP points to a single block of memory

    29 that holds the compiled pattern and associated data. You can find the length of

    30 this block in bytes by calling \fBpcre_fullinfo()\fP with an argument of

    31 PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is

    32 sample code that compiles a pattern and writes it to a file. It assumes that

    33 the variable \fIfd\fP refers to a file that is open for output:

    34 .sp

    35   int erroroffset, rc, size;

    36   char *error;

    37   pcre *re;

    38 .sp

    39   re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);

    40   if (re == NULL) { ... handle errors ... }

    41   rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);

    42   if (rc < 0) { ... handle errors ... }

    43   rc = fwrite(re, 1, size, fd);

    44   if (rc != size) { ... handle errors ... }

    45 .sp

    46 In this example, the bytes that comprise the compiled pattern are copied

    47 exactly. Note that this is binary data that may contain any of the 256 possible

    48 byte values. On systems that make a distinction between binary and non-binary

    49 data, be sure that the file is opened for binary output.

    50 .P

    51 If you want to write more than one pattern to a file, you will have to devise a

    52 way of separating them. For binary data, preceding each pattern with its length

    53 is probably the most straightforward approach. Another possibility is to write

    54 out the data in hexadecimal instead of binary, one pattern to a line.

    55 .P

    56 Saving compiled patterns in a file is only one possible way of storing them for

    57 later use. They could equally well be saved in a database, or in the memory of

    58 some daemon process that passes them via sockets to the processes that want

    59 them.

    60 .P

    61 If the pattern has been studied, it is also possible to save the study data in

    62 a similar way to the compiled pattern itself. When studying generates

    63 additional information, \fBpcre_study()\fP returns a pointer to a

    64 \fBpcre_extra\fP data block. Its format is defined in the

    65 .\" HTML <a href="pcreapi.html#extradata">

    66 .\" </a>

    67 section on matching a pattern

    68 .\"

    69 in the

    70 .\" HREF

    71 \fBpcreapi\fP

    72 .\"

    73 documentation. The \fIstudy_data\fP field points to the binary study data, and

    74 this is what you must save (not the \fBpcre_extra\fP block itself). The length

    75 of the study data can be obtained by calling \fBpcre_fullinfo()\fP with an

    76 argument of PCRE_INFO_STUDYSIZE. Remember to check that \fBpcre_study()\fP did

    77 return a non-NULL value before trying to save the study data.

    78 .

    79 .

    80 .SH "RE-USING A PRECOMPILED PATTERN"

    81 .rs

    82 .sp

    83 Re-using a precompiled pattern is straightforward. Having reloaded it into main

    84 memory, you pass its pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in

    85 the usual way. This should work even on another host, and even if that host has

    86 the opposite endianness to the one where the pattern was compiled.

    87 .P

    88 However, if you passed a pointer to custom character tables when the pattern

    89 was compiled (the \fItableptr\fP argument of \fBpcre_compile()\fP), you must

    90 now pass a similar pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP,

    91 because the value saved with the compiled pattern will obviously be nonsense. A

    92 field in a \fBpcre_extra()\fP block is used to pass this data, as described in

    93 the

    94 .\" HTML <a href="pcreapi.html#extradata">

    95 .\" </a>

    96 section on matching a pattern

    97 .\"

    98 in the

    99 .\" HREF

   100 \fBpcreapi\fP

   101 .\"

   102 documentation.

   103 .P

   104 If you did not provide custom character tables when the pattern was compiled,

   105 the pointer in the compiled pattern is NULL, which causes \fBpcre_exec()\fP to

   106 use PCRE's internal tables. Thus, you do not need to take any special action at

   107 run time in this case.

   108 .P

   109 If you saved study data with the compiled pattern, you need to create your own

   110 \fBpcre_extra\fP data block and set the \fIstudy_data\fP field to point to the

   111 reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the

   112 \fIflags\fP field to indicate that study data is present. Then pass the

   113 \fBpcre_extra\fP block to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in the

   114 usual way.

   115 .

   116 .

   117 .SH "COMPATIBILITY WITH DIFFERENT PCRE RELEASES"

   118 .rs

   119 .sp

   120 In general, it is safest to recompile all saved patterns when you update to a

   121 new PCRE release, though not all updates actually require this. Recompiling is

   122 definitely needed for release 7.2.

   123 .

   124 .

   125 .

   126 .SH AUTHOR

   127 .rs

   128 .sp

   129 .nf

   130 Philip Hazel

   131 University Computing Service

   132 Cambridge CB2 3QH, England.

   133 .fi

   134 .

   135 .

   136 .SH REVISION

   137 .rs

   138 .sp

   139 .nf

   140 Last updated: 13 June 2007

   141 Copyright (c) 1997-2007 University of Cambridge.

   142 .fi