215 lines
9.7 KiB
HTML
215 lines
9.7 KiB
HTML
<html>
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<head>
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<title>pcre2 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>pcre2 man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the 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">INTRODUCTION</a>
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<li><a name="TOC2" href="#SEC2">SECURITY CONSIDERATIONS</a>
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<li><a name="TOC3" href="#SEC3">USER DOCUMENTATION</a>
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<li><a name="TOC4" href="#SEC4">AUTHOR</a>
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<li><a name="TOC5" href="#SEC5">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
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<P>
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PCRE2 is the name used for a revised API for the PCRE library, which is a set
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of functions, written in C, that implement regular expression pattern matching
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using the same syntax and semantics as Perl, with just a few differences. After
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nearly two decades, the limitations of the original API were making development
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increasingly difficult. The new API is more extensible, and it was simplified
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by abolishing the separate "study" optimizing function; in PCRE2, patterns are
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automatically optimized where possible. Since forking from PCRE1, the code has
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been extensively refactored and new features introduced. The old library is now
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obsolete and is no longer maintained.
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</P>
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<P>
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As well as Perl-style regular expression patterns, some features that appeared
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in Python and the original PCRE before they appeared in Perl are available
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using the Python syntax. There is also some support for one or two .NET and
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Oniguruma syntax items, and there are options for requesting some minor changes
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that give better ECMAScript (aka JavaScript) compatibility.
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</P>
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<P>
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The source code for PCRE2 can be compiled to support strings of 8-bit, 16-bit,
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or 32-bit code units, which means that up to three separate libraries may be
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installed, one for each code unit size. The size of code unit is not related to
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the bit size of the underlying hardware. In a 64-bit environment that also
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supports 32-bit applications, versions of PCRE2 that are compiled in both
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64-bit and 32-bit modes may be needed.
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</P>
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<P>
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The original work to extend PCRE to 16-bit and 32-bit code units was done by
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Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings
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can be interpreted either as one character per code unit, or as UTF-encoded
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Unicode, with support for Unicode general category properties. Unicode support
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is optional at build time (but is the default). However, processing strings as
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UTF code units must be enabled explicitly at run time. The version of Unicode
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in use can be discovered by running
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<pre>
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pcre2test -C
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</PRE>
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</P>
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<P>
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The three libraries contain identical sets of functions, with names ending in
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_8, _16, or _32, respectively (for example, <b>pcre2_compile_8()</b>). However,
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by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just
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one code unit width can be written using generic names such as
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<b>pcre2_compile()</b>, and the documentation is written assuming that this is
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the case.
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</P>
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<P>
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In addition to the Perl-compatible matching function, PCRE2 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="pcre2matching.html"><b>pcre2matching</b></a>
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page.
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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 PCRE2 are given in separate documents. See the
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<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
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and
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<a href="pcre2compat.html"><b>pcre2compat</b></a>
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pages. There is a syntax summary in the
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<a href="pcre2syntax.html"><b>pcre2syntax</b></a>
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page.
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</P>
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<P>
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Some features of PCRE2 can be included, excluded, or changed when the library
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is built. The
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<a href="pcre2_config.html"><b>pcre2_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="pcre2build.html"><b>pcre2build</b></a>
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page. Documentation about building PCRE2 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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"_pcre2", which hopefully will not provoke any name clashes. In some
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environments, it is possible to control which external symbols are exported
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when a shared library is built, and in these cases the undocumented symbols are
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not exported.
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</P>
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<br><a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a><br>
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<P>
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If you are using PCRE2 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. For example, an
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8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets
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patterns and subjects as strings of UTF-8 code units instead of individual
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8-bit characters. This causes both the pattern and any data against which it is
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matched to be checked for UTF-8 validity. If the data string is very long, such
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a check might use sufficiently many resources as to cause your application to
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lose 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>pcre2_pattern_info()</b> function to check the compiled pattern's options for
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PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling
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<b>pcre2_compile()</b>. This causes a compile time error if the pattern contains
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a UTF-setting sequence.
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</P>
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<P>
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The use of Unicode properties for character types such as \d can also be
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enabled from within the pattern, by specifying "(*UCP)". This feature can be
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disallowed by setting the PCRE2_NEVER_UCP option.
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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 PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid
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running redundant checks.
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</P>
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<P>
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The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to
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problems, because it may leave the current matching point in the middle of a
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multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an
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application to lock out the use of \C, causing a compile-time error if it is
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encountered. It is also possible to build PCRE2 with the use of \C permanently
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disabled.
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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. PCRE2 provides some protection
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against this: see the <b>pcre2_set_match_limit()</b> function in the
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<a href="pcre2api.html"><b>pcre2api</b></a>
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page. There is a similar function called <b>pcre2_set_depth_limit()</b> that can
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be used to restrict the amount of memory that is used.
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</P>
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<br><a name="SEC3" href="#TOC1">USER DOCUMENTATION</a><br>
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<P>
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The user documentation for PCRE2 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>pcre2grep</b> and <b>pcre2test</b> programs are in
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files called <b>pcre2grep.txt</b> and <b>pcre2test.txt</b>, respectively. The
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remaining sections, except for the <b>pcre2demo</b> section (which is a program
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listing), and the short pages for individual functions, are concatenated in
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<b>pcre2.txt</b>, for ease of searching. The sections are as follows:
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<pre>
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pcre2 this document
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pcre2-config show PCRE2 installation configuration information
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pcre2api details of PCRE2's native C API
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pcre2build building PCRE2
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pcre2callout details of the pattern callout feature
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pcre2compat discussion of Perl compatibility
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pcre2convert details of pattern conversion functions
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pcre2demo a demonstration C program that uses PCRE2
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pcre2grep description of the <b>pcre2grep</b> command (8-bit only)
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pcre2jit discussion of just-in-time optimization support
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pcre2limits details of size and other limits
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pcre2matching discussion of the two matching algorithms
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pcre2partial details of the partial matching facility
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pcre2pattern syntax and semantics of supported regular expression patterns
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pcre2perform discussion of performance issues
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pcre2posix the POSIX-compatible C API for the 8-bit library
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pcre2sample discussion of the pcre2demo program
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pcre2serialize details of pattern serialization
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pcre2syntax quick syntax reference
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pcre2test description of the <b>pcre2test</b> command
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pcre2unicode discussion of Unicode and UTF 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="SEC4" 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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Retired from University Computing Service
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<br>
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Cambridge, 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 is a spam magnet. If you want to email me,
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use my two names separated by a dot at gmail.com.
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</P>
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<br><a name="SEC5" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 27 August 2021
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<br>
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Copyright © 1997-2021 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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