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<html>
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<head>
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<title>pcre specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<ul>
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<li><a name="TOC1" href="#SEC1">PLEASE TAKE NOTE</a>
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<li><a name="TOC2" href="#SEC2">INTRODUCTION</a>
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<li><a name="TOC3" href="#SEC3">SECURITY CONSIDERATIONS</a>
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<li><a name="TOC4" href="#SEC4">USER DOCUMENTATION</a>
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<li><a name="TOC5" href="#SEC5">AUTHOR</a>
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<li><a name="TOC6" href="#SEC6">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PLEASE TAKE NOTE</a><br>
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<P>
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This document relates to PCRE releases that use the original API,
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with library names libpcre, libpcre16, and libpcre32. January 2015 saw the
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first release of a new API, known as PCRE2, with release numbers starting at
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10.00 and library names libpcre2-8, libpcre2-16, and libpcre2-32. The old
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libraries (now called PCRE1) are still being maintained for bug fixes, but
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there will be no new development. New projects are advised to use the new PCRE2
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libraries.
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</P>
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<br><a name="SEC2" href="#TOC1">INTRODUCTION</a><br>
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<P>
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The PCRE library is a set of functions that implement regular expression
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pattern matching using the same syntax and semantics as Perl, with just a few
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differences. Some features that appeared in Python and PCRE before they
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appeared in Perl are also available using the Python syntax, there is some
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support for one or two .NET and Oniguruma syntax items, and there is an option
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for requesting some minor changes that give better JavaScript compatibility.
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</P>
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<P>
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Starting with release 8.30, it is possible to compile two separate PCRE
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libraries: the original, which supports 8-bit character strings (including
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UTF-8 strings), and a second library that supports 16-bit character strings
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(including UTF-16 strings). The build process allows either one or both to be
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built. The majority of the work to make this possible was done by Zoltan
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Herczeg.
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</P>
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<P>
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Starting with release 8.32 it is possible to compile a third separate PCRE
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library that supports 32-bit character strings (including UTF-32 strings). The
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build process allows any combination of the 8-, 16- and 32-bit libraries. The
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work to make this possible was done by Christian Persch.
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</P>
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<P>
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The three libraries contain identical sets of functions, except that the names
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in the 16-bit library start with <b>pcre16_</b> instead of <b>pcre_</b>, and the
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names in the 32-bit library start with <b>pcre32_</b> instead of <b>pcre_</b>. To
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avoid over-complication and reduce the documentation maintenance load, most of
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the documentation describes the 8-bit library, with the differences for the
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16-bit and 32-bit libraries described separately in the
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<a href="pcre16.html"><b>pcre16</b></a>
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and
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<a href="pcre32.html"><b>pcre32</b></a>
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pages. References to functions or structures of the form <i>pcre[16|32]_xxx</i>
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should be read as meaning "<i>pcre_xxx</i> when using the 8-bit library,
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<i>pcre16_xxx</i> when using the 16-bit library, or <i>pcre32_xxx</i> when using
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the 32-bit library".
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</P>
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<P>
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The current implementation of PCRE corresponds approximately with Perl 5.12,
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including support for UTF-8/16/32 encoded strings and Unicode general category
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properties. However, UTF-8/16/32 and Unicode support has to be explicitly
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enabled; it is not the default. The Unicode tables correspond to Unicode
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release 6.3.0.
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</P>
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<P>
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In addition to the Perl-compatible matching function, PCRE contains an
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alternative function that matches the same compiled patterns in a different
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way. In certain circumstances, the alternative function has some advantages.
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For a discussion of the two matching algorithms, see the
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<a href="pcrematching.html"><b>pcrematching</b></a>
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page.
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</P>
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<P>
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PCRE is written in C and released as a C library. A number of people have
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written wrappers and interfaces of various kinds. In particular, Google Inc.
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have provided a comprehensive C++ wrapper for the 8-bit library. This is now
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included as part of the PCRE distribution. The
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<a href="pcrecpp.html"><b>pcrecpp</b></a>
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page has details of this interface. Other people's contributions can be found
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in the <i>Contrib</i> directory at the primary FTP site, which is:
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<a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
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</P>
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<P>
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Details of exactly which Perl regular expression features are and are not
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supported by PCRE are given in separate documents. See the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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and
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<a href="pcrecompat.html"><b>pcrecompat</b></a>
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pages. There is a syntax summary in the
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<a href="pcresyntax.html"><b>pcresyntax</b></a>
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page.
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</P>
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<P>
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Some features of PCRE can be included, excluded, or changed when the library is
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built. The
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<a href="pcre_config.html"><b>pcre_config()</b></a>
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function makes it possible for a client to discover which features are
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available. The features themselves are described in the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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page. Documentation about building PCRE for various operating systems can be
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found in the
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<a href="README.txt"><b>README</b></a>
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and
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<a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a>
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files in the source distribution.
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</P>
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<P>
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The libraries contains a number of undocumented internal functions and data
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tables that are used by more than one of the exported external functions, but
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which are not intended for use by external callers. Their names all begin with
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"_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name
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clashes. In some environments, it is possible to control which external symbols
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are exported when a shared library is built, and in these cases the
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undocumented symbols are not exported.
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</P>
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<br><a name="SEC3" href="#TOC1">SECURITY CONSIDERATIONS</a><br>
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<P>
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If you are using PCRE in a non-UTF application that permits users to supply
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arbitrary patterns for compilation, you should be aware of a feature that
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allows users to turn on UTF support from within a pattern, provided that PCRE
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was built with UTF support. For example, an 8-bit pattern that begins with
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"(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and
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subjects as strings of UTF-8 characters instead of individual 8-bit characters.
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This causes both the pattern and any data against which it is matched to be
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checked for UTF-8 validity. If the data string is very long, such a check might
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use sufficiently many resources as to cause your application to lose
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performance.
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</P>
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<P>
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One way of guarding against this possibility is to use the
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<b>pcre_fullinfo()</b> function to check the compiled pattern's options for UTF.
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Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF option at
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compile time. This causes a compile time error if a pattern contains a
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UTF-setting sequence.
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</P>
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<P>
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If your application is one that supports UTF, be aware that validity checking
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can take time. If the same data string is to be matched many times, you can use
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the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to
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save redundant checks.
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</P>
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<P>
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Another way that performance can be hit is by running a pattern that has a very
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large search tree against a string that will never match. Nested unlimited
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repeats in a pattern are a common example. PCRE provides some protection
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against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the
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<a href="pcreapi.html"><b>pcreapi</b></a>
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page.
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</P>
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<br><a name="SEC4" href="#TOC1">USER DOCUMENTATION</a><br>
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<P>
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The user documentation for PCRE comprises a number of different sections. In
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the "man" format, each of these is a separate "man page". In the HTML format,
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each is a separate page, linked from the index page. In the plain text format,
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the descriptions of the <b>pcregrep</b> and <b>pcretest</b> programs are in files
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called <b>pcregrep.txt</b> and <b>pcretest.txt</b>, respectively. The remaining
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sections, except for the <b>pcredemo</b> section (which is a program listing),
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are concatenated in <b>pcre.txt</b>, for ease of searching. The sections are as
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follows:
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<pre>
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pcre this document
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pcre-config show PCRE installation configuration information
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pcre16 details of the 16-bit library
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pcre32 details of the 32-bit library
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pcreapi details of PCRE's native C API
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pcrebuild building PCRE
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pcrecallout details of the callout feature
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pcrecompat discussion of Perl compatibility
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pcrecpp details of the C++ wrapper for the 8-bit library
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pcredemo a demonstration C program that uses PCRE
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pcregrep description of the <b>pcregrep</b> command (8-bit only)
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pcrejit discussion of the just-in-time optimization support
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pcrelimits details of size and other limits
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pcrematching discussion of the two matching algorithms
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pcrepartial details of the partial matching facility
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pcrepattern syntax and semantics of supported regular expressions
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pcreperform discussion of performance issues
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pcreposix the POSIX-compatible C API for the 8-bit library
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pcreprecompile details of saving and re-using precompiled patterns
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pcresample discussion of the pcredemo program
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pcrestack discussion of stack usage
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pcresyntax quick syntax reference
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pcretest description of the <b>pcretest</b> testing command
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pcreunicode discussion of Unicode and UTF-8/16/32 support
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</pre>
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In the "man" and HTML formats, there is also a short page for each C library
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function, listing its arguments and results.
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</P>
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<br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<P>
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Putting an actual email address here seems to have been a spam magnet, so I've
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taken it away. If you want to email me, use my two initials, followed by the
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two digits 10, at the domain cam.ac.uk.
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</P>
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<br><a name="SEC6" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 10 February 2015
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<br>
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Copyright © 1997-2015 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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