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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">INTRODUCTION</a>
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<li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
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<li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
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<li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</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">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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The current implementation of PCRE corresponds approximately with Perl 5.10,
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including support for UTF-8 encoded strings and Unicode general category
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properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
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is not the default. The Unicode tables correspond to Unicode release 5.1.
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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. This is now included as part of the
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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 <b>README</b> and <b>NON-UNIX-USE</b> files in the source
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distribution.
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</P>
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<P>
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The library 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_", 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">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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all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
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of searching. The sections are as 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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pcreapi details of PCRE's native C API
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pcrebuild options for 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
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pcredemo a demonstration C program that uses PCRE
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pcregrep description of the <b>pcregrep</b> command
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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
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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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</pre>
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In addition, in the "man" and HTML formats, there is a short page for each
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C library function, listing its arguments and results.
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</P>
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<br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
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<P>
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There are some size limitations in PCRE but it is hoped that they will never in
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practice be relevant.
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</P>
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<P>
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The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
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compiled with the default internal linkage size of 2. If you want to process
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regular expressions that are truly enormous, you can compile PCRE with an
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internal linkage size of 3 or 4 (see the <b>README</b> file in the source
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distribution and the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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documentation for details). In these cases the limit is substantially larger.
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However, the speed of execution is slower.
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</P>
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<P>
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All values in repeating quantifiers must be less than 65536.
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</P>
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<P>
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There is no limit to the number of parenthesized subpatterns, but there can be
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no more than 65535 capturing subpatterns.
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</P>
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<P>
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The maximum length of name for a named subpattern is 32 characters, and the
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maximum number of named subpatterns is 10000.
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</P>
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<P>
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The maximum length of a subject string is the largest positive number that an
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integer variable can hold. However, when using the traditional matching
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function, PCRE uses recursion to handle subpatterns and indefinite repetition.
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This means that the available stack space may limit the size of a subject
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string that can be processed by certain patterns. For a discussion of stack
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issues, see the
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<a href="pcrestack.html"><b>pcrestack</b></a>
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documentation.
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<a name="utf8support"></a></P>
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<br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
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<P>
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From release 3.3, PCRE has had some support for character strings encoded in
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the UTF-8 format. For release 4.0 this was greatly extended to cover most
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common requirements, and in release 5.0 additional support for Unicode general
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category properties was added.
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</P>
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<P>
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In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
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the code, and, in addition, you must call
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<a href="pcre_compile.html"><b>pcre_compile()</b></a>
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with the PCRE_UTF8 option flag, or the pattern must start with the sequence
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(*UTF8). When either of these is the case, both the pattern and any subject
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strings that are matched against it are treated as UTF-8 strings instead of
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strings of 1-byte characters.
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</P>
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<P>
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If you compile PCRE with UTF-8 support, but do not use it at run time, the
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library will be a bit bigger, but the additional run time overhead is limited
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to testing the PCRE_UTF8 flag occasionally, so should not be very big.
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</P>
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<P>
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If PCRE is built with Unicode character property support (which implies UTF-8
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support), the escape sequences \p{..}, \P{..}, and \X are supported.
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The available properties that can be tested are limited to the general
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category properties such as Lu for an upper case letter or Nd for a decimal
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number, the Unicode script names such as Arabic or Han, and the derived
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properties Any and L&. A full list is given in the
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<a href="pcrepattern.html"><b>pcrepattern</b></a>
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documentation. Only the short names for properties are supported. For example,
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\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
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Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
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compatibility with Perl 5.6. PCRE does not support this.
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<a name="utf8strings"></a></P>
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<br><b>
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Validity of UTF-8 strings
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</b><br>
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<P>
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When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
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are (by default) checked for validity on entry to the relevant functions. From
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release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
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themselves derived from the Unicode specification. Earlier releases of PCRE
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followed the rules of RFC 2279, which allows the full range of 31-bit values (0
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to 0x7FFFFFFF). The current check allows only values in the range U+0 to
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U+10FFFF, excluding U+D800 to U+DFFF.
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</P>
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<P>
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The excluded code points are the "Low Surrogate Area" of Unicode, of which the
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Unicode Standard says this: "The Low Surrogate Area does not contain any
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character assignments, consequently no character code charts or namelists are
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provided for this area. Surrogates are reserved for use with UTF-16 and then
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must be used in pairs." The code points that are encoded by UTF-16 pairs are
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available as independent code points in the UTF-8 encoding. (In other words,
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the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
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UTF-8.)
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</P>
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<P>
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If an invalid UTF-8 string is passed to PCRE, an error return
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(PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
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your strings are valid, and therefore want to skip these checks in order to
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improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
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at run time, PCRE assumes that the pattern or subject it is given
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(respectively) contains only valid UTF-8 codes. In this case, it does not
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diagnose an invalid UTF-8 string.
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</P>
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<P>
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If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
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happens depends on why the string is invalid. If the string conforms to the
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"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
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in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
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test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
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rules of RFC 2279. However, if the string does not even conform to RFC 2279,
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the result is undefined. Your program may crash.
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</P>
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<P>
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If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
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encoded in a UTF-8-like manner as per the old RFC, you can set
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PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
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situation, you will have to apply your own validity check.
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</P>
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<br><b>
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General comments about UTF-8 mode
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</b><br>
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<P>
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1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
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UTF-8 character if the value is greater than 127.
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</P>
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<P>
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2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
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characters for values greater than \177.
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</P>
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<P>
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3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
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bytes, for example: \x{100}{3}.
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</P>
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<P>
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4. The dot metacharacter matches one UTF-8 character instead of a single byte.
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</P>
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<P>
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5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
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but its use can lead to some strange effects. This facility is not available in
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the alternative matching function, <b>pcre_dfa_exec()</b>.
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</P>
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<P>
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6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
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test characters of any code value, but the characters that PCRE recognizes as
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digits, spaces, or word characters remain the same set as before, all with
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values less than 256. This remains true even when PCRE includes Unicode
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property support, because to do otherwise would slow down PCRE in many common
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cases. If you really want to test for a wider sense of, say, "digit", you
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must use Unicode property tests such as \p{Nd}. Note that this also applies to
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\b, because it is defined in terms of \w and \W.
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</P>
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<P>
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7. Similarly, characters that match the POSIX named character classes are all
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low-valued characters.
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</P>
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<P>
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8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
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(\h, \H, \v, and \V) do match all the appropriate Unicode characters.
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</P>
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<P>
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9. Case-insensitive matching applies only to characters whose values are less
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than 128, unless PCRE is built with Unicode property support. Even when Unicode
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property support is available, PCRE still uses its own character tables when
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checking the case of low-valued characters, so as not to degrade performance.
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The Unicode property information is used only for characters with higher
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values. Even when Unicode property support is available, PCRE supports
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case-insensitive matching only when there is a one-to-one mapping between a
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letter's cases. There are a small number of many-to-one mappings in Unicode;
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these are not supported by PCRE.
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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: 28 September 2009
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<br>
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Copyright © 1997-2009 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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