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    1 <html>
    2 <head>
    3 <title>pcre specification</title>
    4 </head>
    5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
    6 <h1>pcre man page</h1>
    7 <p>
    8 Return to the <a href="index.html">PCRE index page</a>.
    9 </p>
   10 <p>
   11 This page is part of the PCRE HTML documentation. It was generated automatically
   12 from the original man page. If there is any nonsense in it, please consult the
   13 man page, in case the conversion went wrong.
   14 <br>
   15 <ul>
   16 <li><a name="TOC1" href="#SEC1">PLEASE TAKE NOTE</a>
   17 <li><a name="TOC2" href="#SEC2">INTRODUCTION</a>
   18 <li><a name="TOC3" href="#SEC3">SECURITY CONSIDERATIONS</a>
   19 <li><a name="TOC4" href="#SEC4">USER DOCUMENTATION</a>
   20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
   21 <li><a name="TOC6" href="#SEC6">REVISION</a>
   22 </ul>
   23 <br><a name="SEC1" href="#TOC1">PLEASE TAKE NOTE</a><br>
   24 <P>
   25 This document relates to PCRE releases that use the original API,
   26 with library names libpcre, libpcre16, and libpcre32. January 2015 saw the
   27 first release of a new API, known as PCRE2, with release numbers starting at
   28 10.00 and library names libpcre2-8, libpcre2-16, and libpcre2-32. The old
   29 libraries (now called PCRE1) are still being maintained for bug fixes, but
   30 there will be no new development. New projects are advised to use the new PCRE2
   31 libraries.
   32 </P>
   33 <br><a name="SEC2" href="#TOC1">INTRODUCTION</a><br>
   34 <P>
   35 The PCRE library is a set of functions that implement regular expression
   36 pattern matching using the same syntax and semantics as Perl, with just a few
   37 differences. Some features that appeared in Python and PCRE before they
   38 appeared in Perl are also available using the Python syntax, there is some
   39 support for one or two .NET and Oniguruma syntax items, and there is an option
   40 for requesting some minor changes that give better JavaScript compatibility.
   41 </P>
   42 <P>
   43 Starting with release 8.30, it is possible to compile two separate PCRE
   44 libraries: the original, which supports 8-bit character strings (including
   45 UTF-8 strings), and a second library that supports 16-bit character strings
   46 (including UTF-16 strings). The build process allows either one or both to be
   47 built. The majority of the work to make this possible was done by Zoltan
   48 Herczeg.
   49 </P>
   50 <P>
   51 Starting with release 8.32 it is possible to compile a third separate PCRE
   52 library that supports 32-bit character strings (including UTF-32 strings). The
   53 build process allows any combination of the 8-, 16- and 32-bit libraries. The
   54 work to make this possible was done by Christian Persch.
   55 </P>
   56 <P>
   57 The three libraries contain identical sets of functions, except that the names
   58 in the 16-bit library start with <b>pcre16_</b> instead of <b>pcre_</b>, and the
   59 names in the 32-bit library start with <b>pcre32_</b> instead of <b>pcre_</b>. To
   60 avoid over-complication and reduce the documentation maintenance load, most of
   61 the documentation describes the 8-bit library, with the differences for the
   62 16-bit and 32-bit libraries described separately in the
   63 <a href="pcre16.html"><b>pcre16</b></a>
   64 and
   65 <a href="pcre32.html"><b>pcre32</b></a>
   66 pages. References to functions or structures of the form <i>pcre[16|32]_xxx</i>
   67 should be read as meaning "<i>pcre_xxx</i> when using the 8-bit library,
   68 <i>pcre16_xxx</i> when using the 16-bit library, or <i>pcre32_xxx</i> when using
   69 the 32-bit library".
   70 </P>
   71 <P>
   72 The current implementation of PCRE corresponds approximately with Perl 5.12,
   73 including support for UTF-8/16/32 encoded strings and Unicode general category
   74 properties. However, UTF-8/16/32 and Unicode support has to be explicitly
   75 enabled; it is not the default. The Unicode tables correspond to Unicode
   76 release 6.3.0.
   77 </P>
   78 <P>
   79 In addition to the Perl-compatible matching function, PCRE contains an
   80 alternative function that matches the same compiled patterns in a different
   81 way. In certain circumstances, the alternative function has some advantages.
   82 For a discussion of the two matching algorithms, see the
   83 <a href="pcrematching.html"><b>pcrematching</b></a>
   84 page.
   85 </P>
   86 <P>
   87 PCRE is written in C and released as a C library. A number of people have
   88 written wrappers and interfaces of various kinds. In particular, Google Inc.
   89 have provided a comprehensive C++ wrapper for the 8-bit library. This is now
   90 included as part of the PCRE distribution. The
   91 <a href="pcrecpp.html"><b>pcrecpp</b></a>
   92 page has details of this interface. Other people's contributions can be found
   93 in the <i>Contrib</i> directory at the primary FTP site, which is:
   94 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
   95 </P>
   96 <P>
   97 Details of exactly which Perl regular expression features are and are not
   98 supported by PCRE are given in separate documents. See the
   99 <a href="pcrepattern.html"><b>pcrepattern</b></a>
  100 and
  101 <a href="pcrecompat.html"><b>pcrecompat</b></a>
  102 pages. There is a syntax summary in the
  103 <a href="pcresyntax.html"><b>pcresyntax</b></a>
  104 page.
  105 </P>
  106 <P>
  107 Some features of PCRE can be included, excluded, or changed when the library is
  108 built. The
  109 <a href="pcre_config.html"><b>pcre_config()</b></a>
  110 function makes it possible for a client to discover which features are
  111 available. The features themselves are described in the
  112 <a href="pcrebuild.html"><b>pcrebuild</b></a>
  113 page. Documentation about building PCRE for various operating systems can be
  114 found in the
  115 <a href="README.txt"><b>README</b></a>
  116 and
  117 <a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a>
  118 files in the source distribution.
  119 </P>
  120 <P>
  121 The libraries contains a number of undocumented internal functions and data
  122 tables that are used by more than one of the exported external functions, but
  123 which are not intended for use by external callers. Their names all begin with
  124 "_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name
  125 clashes. In some environments, it is possible to control which external symbols
  126 are exported when a shared library is built, and in these cases the
  127 undocumented symbols are not exported.
  128 </P>
  129 <br><a name="SEC3" href="#TOC1">SECURITY CONSIDERATIONS</a><br>
  130 <P>
  131 If you are using PCRE in a non-UTF application that permits users to supply
  132 arbitrary patterns for compilation, you should be aware of a feature that
  133 allows users to turn on UTF support from within a pattern, provided that PCRE
  134 was built with UTF support. For example, an 8-bit pattern that begins with
  135 "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and
  136 subjects as strings of UTF-8 characters instead of individual 8-bit characters.
  137 This causes both the pattern and any data against which it is matched to be
  138 checked for UTF-8 validity. If the data string is very long, such a check might
  139 use sufficiently many resources as to cause your application to lose
  140 performance.
  141 </P>
  142 <P>
  143 One way of guarding against this possibility is to use the
  144 <b>pcre_fullinfo()</b> function to check the compiled pattern's options for UTF.
  145 Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF option at
  146 compile time. This causes a compile time error if a pattern contains a
  147 UTF-setting sequence.
  148 </P>
  149 <P>
  150 If your application is one that supports UTF, be aware that validity checking
  151 can take time. If the same data string is to be matched many times, you can use
  152 the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to
  153 save redundant checks.
  154 </P>
  155 <P>
  156 Another way that performance can be hit is by running a pattern that has a very
  157 large search tree against a string that will never match. Nested unlimited
  158 repeats in a pattern are a common example. PCRE provides some protection
  159 against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the
  160 <a href="pcreapi.html"><b>pcreapi</b></a>
  161 page.
  162 </P>
  163 <br><a name="SEC4" href="#TOC1">USER DOCUMENTATION</a><br>
  164 <P>
  165 The user documentation for PCRE comprises a number of different sections. In
  166 the "man" format, each of these is a separate "man page". In the HTML format,
  167 each is a separate page, linked from the index page. In the plain text format,
  168 the descriptions of the <b>pcregrep</b> and <b>pcretest</b> programs are in files
  169 called <b>pcregrep.txt</b> and <b>pcretest.txt</b>, respectively. The remaining
  170 sections, except for the <b>pcredemo</b> section (which is a program listing),
  171 are concatenated in <b>pcre.txt</b>, for ease of searching. The sections are as
  172 follows:
  173 <pre>
  174   pcre              this document
  175   pcre-config       show PCRE installation configuration information
  176   pcre16            details of the 16-bit library
  177   pcre32            details of the 32-bit library
  178   pcreapi           details of PCRE's native C API
  179   pcrebuild         building PCRE
  180   pcrecallout       details of the callout feature
  181   pcrecompat        discussion of Perl compatibility
  182   pcrecpp           details of the C++ wrapper for the 8-bit library
  183   pcredemo          a demonstration C program that uses PCRE
  184   pcregrep          description of the <b>pcregrep</b> command (8-bit only)
  185   pcrejit           discussion of the just-in-time optimization support
  186   pcrelimits        details of size and other limits
  187   pcrematching      discussion of the two matching algorithms
  188   pcrepartial       details of the partial matching facility
  189   pcrepattern       syntax and semantics of supported regular expressions
  190   pcreperform       discussion of performance issues
  191   pcreposix         the POSIX-compatible C API for the 8-bit library
  192   pcreprecompile    details of saving and re-using precompiled patterns
  193   pcresample        discussion of the pcredemo program
  194   pcrestack         discussion of stack usage
  195   pcresyntax        quick syntax reference
  196   pcretest          description of the <b>pcretest</b> testing command
  197   pcreunicode       discussion of Unicode and UTF-8/16/32 support
  198 </pre>
  199 In the "man" and HTML formats, there is also a short page for each C library
  200 function, listing its arguments and results.
  201 </P>
  202 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
  203 <P>
  204 Philip Hazel
  205 <br>
  206 University Computing Service
  207 <br>
  208 Cambridge CB2 3QH, England.
  209 <br>
  210 </P>
  211 <P>
  212 Putting an actual email address here seems to have been a spam magnet, so I've
  213 taken it away. If you want to email me, use my two initials, followed by the
  214 two digits 10, at the domain cam.ac.uk.
  215 </P>
  216 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
  217 <P>
  218 Last updated: 10 February 2015
  219 <br>
  220 Copyright &copy; 1997-2015 University of Cambridge.
  221 <br>
  222 <p>
  223 Return to the <a href="index.html">PCRE index page</a>.
  224 </p>