doc/html/pcre2.html

<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="Asciidoctor 2.0.26"><meta name="description" content="Introductory page"><title>pcre2(3)</title><link rel="stylesheet" href="./css/document.css"><link rel="stylesheet" href="./css/standalone.css"><meta name="color-scheme" content="light dark"><meta name="theme-color" media="(prefers-color-scheme: light)" content="#0085cc"><meta name="theme-color" media="(prefers-color-scheme: dark)" content="#33b8ff"></head><body class="manpage offline-manual"><header class="offline-nav"><nav><a class="offline-index" href="./index.html">Index</a><a class="offline-home" href="https://www.pcre2.org/"><span aria-hidden="true">🌐︎</span> PCRE2 homepage</a></nav></header><main class="document-page"><header class="document-title"><h1>pcre2(3)</h1><p class="document-meta">PCRE2 10.50-DEV manual; updated <time datetime="2025-02-22">22 Feb 2025</time></p></header><div class="document-layout"><article class="prose" id="content"><section class="doc-section level-1"><h2 id="_introduction">Introduction</h2><p>PCRE2 is the name used for a revised API for the PCRE library, which is a set
of functions, written in C, that implement regular expression pattern matching
using the same syntax and semantics as Perl, with just a few differences. After
nearly two decades, the limitations of the original API were making development
increasingly difficult. The new API is more extensible, and it was simplified
by abolishing the separate "study" optimizing function; in PCRE2, patterns are
automatically optimized where possible. Since forking from PCRE1, the code has
been extensively refactored and new features introduced. The old library is now
obsolete and is no longer maintained.</p>
<p>As well as Perl-style regular expression patterns, some features that appeared
in Python and the original PCRE before they appeared in Perl are available
using the Python syntax. There is also support for some .NET and Oniguruma
syntax items, and there are options for requesting minor changes that give
better ECMAScript (JavaScript) compatibility.</p>
<p>The source code for PCRE2 can be compiled to support strings of 8-bit, 16-bit,
or 32-bit code units, which means that up to three separate libraries may be
installed, one for each code unit size. The size of a code unit is not related
to the bit size of the underlying hardware. In a 64-bit environment that also
supports 32-bit applications, versions of PCRE2 that are compiled in both
64-bit and 32-bit modes may be needed.</p>
<p>The original work to extend PCRE to 16-bit and 32-bit code units was done by
Zoltán Herczeg and Christian Persch, respectively. In all three cases, strings
can be interpreted either as one character per code unit, or as UTF-encoded
Unicode, with support for Unicode general category properties. Unicode support
is optional at build time (but is the default). However, processing strings as
UTF code units must be enabled explicitly at run time. The version of Unicode
in use can be discovered by running</p>
<div class="listing-block"><pre class="rouge highlight"><code data-lang="shell">pcre2test <span class="nt">-C</span></code></pre></div>
<p>The three libraries contain identical sets of functions, with names ending in
<code>_8</code>, <code>_16</code>, or <code>_32</code>, respectively (for example, <code class="function pcre2-api rouge"><span class="nf">pcre2_compile_8()</span></code>). However,
by defining <code class="constant pcre2-api rouge"><span class="no">PCRE2_CODE_UNIT_WIDTH</span></code> to be 8, 16, or 32, a program that uses just
one code unit width can be written using generic names such as
<code class="function pcre2-api rouge"><span class="nf">pcre2_compile()</span></code>, and the documentation is written assuming that this is
the case.</p>
<p>In addition to the Perl-compatible matching function, PCRE2 contains an
alternative 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
<a href="./pcre2matching.html"><code>pcre2matching</code></a>
page.</p>
<p>Details of exactly which Perl regular expression features are and are not
supported by PCRE2 are given in separate documents. See the
<a href="./pcre2pattern.html"><code>pcre2pattern</code></a>
and
<a href="./pcre2compat.html"><code>pcre2compat</code></a>
pages. There is a syntax summary in the
<a href="./pcre2syntax.html"><code>pcre2syntax</code></a>
page.</p>
<p>Some features of PCRE2 can be included, excluded, or changed when the library
is built. The <code class="function pcre2-api rouge"><span class="nf">pcre2_config()</span></code>
function makes it possible for a client to discover which features are
available. The features themselves are described in the
<a href="./pcre2build.html"><code>pcre2build</code></a>
page. Documentation about building PCRE2 for various operating systems can be
found in the
<code>README.md</code>
and
<code>NON-AUTOTOOLS-BUILD.md</code>
files in the source distribution.</p>
<p>The libraries 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
<code class="c rouge" data-lang="c"><span class="nv">_pcre2</span></code>, 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.</p></section>
<section class="doc-section level-1"><h2 id="security-considerations">Security considerations</h2><p>If you are using PCRE2 in a non-UTF application that permits users to supply
arbitrary patterns for compilation, you should be aware of a feature that
allows users to turn on UTF support from within a pattern. For example, an
8-bit pattern that begins with <code class="regex rouge"><span class="pcre2-semantic">(*UTF)</span></code> turns on UTF-8 mode, which interprets
patterns and subjects as strings of UTF-8 code units instead of individual
8-bit characters. This causes both the pattern and any data against which it is
matched to be checked for UTF-8 validity. If the data string is very long, such
a check might use sufficiently many resources as to cause your application to
lose performance.</p>
<p>One way of guarding against this possibility is to use the
<code class="function pcre2-api rouge"><span class="nf">pcre2_pattern_info()</span></code> function to check the compiled pattern’s options for
<code class="constant pcre2-api rouge"><span class="no">PCRE2_UTF</span></code>. Alternatively, you can set the <code class="constant pcre2-api rouge"><span class="no">PCRE2_NEVER_UTF</span></code> option when calling
<code class="function pcre2-api rouge"><span class="nf">pcre2_compile()</span></code>. This causes a compile time error if the pattern contains
a UTF-setting sequence.</p>
<p>The use of Unicode properties for character types such as <code class="regex rouge"><span class="pcre2-semantic">\d</span></code> can also be
enabled from within the pattern, by specifying <code class="regex rouge"><span class="pcre2-semantic">(*UCP)</span></code>. This feature can be
disallowed by setting the <code class="constant pcre2-api rouge"><span class="no">PCRE2_NEVER_UCP</span></code> option.</p>
<p>If your application is one that supports UTF, be aware that validity checking
can take time. If the same data string is to be matched many times, you can use
the <code class="constant pcre2-api rouge"><span class="no">PCRE2_NO_UTF_CHECK</span></code> option for the second and subsequent matches to avoid
running redundant checks.</p>
<p>The use of the <code class="regex rouge"><span class="pcre2-semantic">\C</span></code> escape sequence in a UTF-8 or UTF-16 pattern can lead to
problems, because it may leave the current matching point in the middle of a
multi-code-unit character. The <code class="constant pcre2-api rouge"><span class="no">PCRE2_NEVER_BACKSLASH_C</span></code> option can be used by an
application to lock out the use of <code class="regex rouge"><span class="pcre2-semantic">\C</span></code>, causing a compile-time error if it is
encountered. It is also possible to build PCRE2 with the use of <code class="regex rouge"><span class="pcre2-semantic">\C</span></code> permanently
disabled.</p>
<p>Another way that performance can be hit is by running a pattern that has a very
large search tree against a string that will never match. Nested unlimited
repeats in a pattern are a common example. PCRE2 provides some protection
against this: see the <code class="function pcre2-api rouge"><span class="nf">pcre2_set_match_limit()</span></code> function in the
<a href="./pcre2api.html"><code>pcre2api</code></a>
page. There is a similar function called <code class="function pcre2-api rouge"><span class="nf">pcre2_set_depth_limit()</span></code> that can
be used to restrict the amount of memory that is used.</p></section>
<section class="doc-section level-1"><h2 id="_user_documentation">User documentation</h2><p>The user documentation for PCRE2 comprises a number of different sections. 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,
the descriptions of the <code>pcre2grep</code> and <code>pcre2test</code> programs are in
files called <code>pcre2grep.txt</code> and <code>pcre2test.txt</code>, respectively. The
remaining sections, except for the <code>pcre2demo</code> section (which is a program
listing), and the short pages for individual functions, are concatenated in
<code>pcre2.txt</code>, for ease of searching. The sections are as follows:</p>
<dl class="values compact" style="--tab-width: 8"><div class="dlist-item"><dt><span>pcre2</span></dt><dd>this document</dd></div><div class="dlist-item"><dt><span>pcre2-config</span></dt><dd>show PCRE2 installation configuration information</dd></div><div class="dlist-item"><dt><span>pcre2api</span></dt><dd>details of PCRE2’s native C API</dd></div><div class="dlist-item"><dt><span>pcre2build</span></dt><dd>building PCRE2</dd></div><div class="dlist-item"><dt><span>pcre2callout</span></dt><dd>details of the pattern callout feature</dd></div><div class="dlist-item"><dt><span>pcre2compat</span></dt><dd>discussion of Perl compatibility</dd></div><div class="dlist-item"><dt><span>pcre2convert</span></dt><dd>details of pattern conversion functions</dd></div><div class="dlist-item"><dt><span>pcre2demo</span></dt><dd>a demonstration C program that uses PCRE2</dd></div><div class="dlist-item"><dt><span>pcre2grep</span></dt><dd>description of the <code>pcre2grep</code> command (8-bit only)</dd></div><div class="dlist-item"><dt><span>pcre2jit</span></dt><dd>discussion of just-in-time optimization support</dd></div><div class="dlist-item"><dt><span>pcre2limits</span></dt><dd>details of size and other limits</dd></div><div class="dlist-item"><dt><span>pcre2matching</span></dt><dd>discussion of the two matching algorithms</dd></div><div class="dlist-item"><dt><span>pcre2partial</span></dt><dd>details of the partial matching facility</dd></div><div class="dlist-item"><dt><span>pcre2pattern</span></dt><dd>syntax and semantics of supported regular
expression patterns</dd></div><div class="dlist-item"><dt><span>pcre2perform</span></dt><dd>discussion of performance issues</dd></div><div class="dlist-item"><dt><span>pcre2posix</span></dt><dd>the POSIX-compatible C API for the 8-bit library</dd></div><div class="dlist-item"><dt><span>pcre2sample</span></dt><dd>discussion of the pcre2demo program</dd></div><div class="dlist-item"><dt><span>pcre2serialize</span></dt><dd>details of pattern serialization</dd></div><div class="dlist-item"><dt><span>pcre2syntax</span></dt><dd>quick syntax reference</dd></div><div class="dlist-item"><dt><span>pcre2test</span></dt><dd>description of the <code>pcre2test</code> command</dd></div><div class="dlist-item"><dt><span>pcre2unicode</span></dt><dd>discussion of Unicode and UTF support</dd></div></dl>
<p>In the "man" and HTML formats, there is also a short page for each C library
function, listing its arguments and results.</p>
<h2 id="_authors" class="discrete">Authors</h2>
<p>The current maintainers of PCRE2 are Nicholas Wilson and Zoltán Herczeg.</p>
<p>PCRE2 was written by Philip Hazel, of the University Computing Service,
Cambridge, England. Many others have also contributed.</p>
<p>To contact the maintainers, please use the GitHub issues tracker or PCRE2
mailing list, as described at the project page:
<a class="bare" href="https://github.com/PCRE2Project/pcre2">https://github.com/PCRE2Project/pcre2</a></p>
<h2 id="_revision" class="discrete">Revision</h2>
<p>Last updated: 22 February 2025<br>
Copyright © 1997-2021 University of Cambridge.</p></section></article></div></main><footer class="offline-footer"></footer></body></html>