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1<html>
2<head>
3<title>pcre2 specification</title>
4</head>
5<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6<h1>pcre2 man page</h1>
7<p>
8Return to the <a href="index.html">PCRE2 index page</a>.
9</p>
10<p>
11This page is part of the PCRE2 HTML documentation. It was generated
12automatically from the original man page. If there is any nonsense in it,
13please consult the man page, in case the conversion went wrong.
14<br>
15<ul>
16<li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17<li><a name="TOC2" href="#SEC2">SECURITY CONSIDERATIONS</a>
18<li><a name="TOC3" href="#SEC3">USER DOCUMENTATION</a>
19<li><a name="TOC4" href="#SEC4">AUTHOR</a>
20<li><a name="TOC5" href="#SEC5">REVISION</a>
21</ul>
22<br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23<P>
24PCRE2 is the name used for a revised API for the PCRE library, which is a set
25of functions, written in C, that implement regular expression pattern matching
26using the same syntax and semantics as Perl, with just a few differences. Some
27features that appeared in Python and the original PCRE before they appeared in
28Perl are also available using the Python syntax. There is also some support for
29one or two .NET and Oniguruma syntax items, and there are options for
30requesting some minor changes that give better ECMAScript (aka JavaScript)
31compatibility.
32</P>
33<P>
34The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit
35code units, which means that up to three separate libraries may be installed.
36The original work to extend PCRE to 16-bit and 32-bit code units was done by
37Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings
38can be interpreted either as one character per code unit, or as UTF-encoded
39Unicode, with support for Unicode general category properties. Unicode support
40is optional at build time (but is the default). However, processing strings as
41UTF code units must be enabled explicitly at run time. The version of Unicode
42in use can be discovered by running
43<pre>
44  pcre2test -C
45</PRE>
46</P>
47<P>
48The three libraries contain identical sets of functions, with names ending in
49_8, _16, or _32, respectively (for example, <b>pcre2_compile_8()</b>). However,
50by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just
51one code unit width can be written using generic names such as
52<b>pcre2_compile()</b>, and the documentation is written assuming that this is
53the case.
54</P>
55<P>
56In addition to the Perl-compatible matching function, PCRE2 contains an
57alternative function that matches the same compiled patterns in a different
58way. In certain circumstances, the alternative function has some advantages.
59For a discussion of the two matching algorithms, see the
60<a href="pcre2matching.html"><b>pcre2matching</b></a>
61page.
62</P>
63<P>
64Details of exactly which Perl regular expression features are and are not
65supported by PCRE2 are given in separate documents. See the
66<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
67and
68<a href="pcre2compat.html"><b>pcre2compat</b></a>
69pages. There is a syntax summary in the
70<a href="pcre2syntax.html"><b>pcre2syntax</b></a>
71page.
72</P>
73<P>
74Some features of PCRE2 can be included, excluded, or changed when the library
75is built. The
76<a href="pcre2_config.html"><b>pcre2_config()</b></a>
77function makes it possible for a client to discover which features are
78available. The features themselves are described in the
79<a href="pcre2build.html"><b>pcre2build</b></a>
80page. Documentation about building PCRE2 for various operating systems can be
81found in the
82<a href="README.txt"><b>README</b></a>
83and
84<a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a>
85files in the source distribution.
86</P>
87<P>
88The libraries contains a number of undocumented internal functions and data
89tables that are used by more than one of the exported external functions, but
90which are not intended for use by external callers. Their names all begin with
91"_pcre2", which hopefully will not provoke any name clashes. In some
92environments, it is possible to control which external symbols are exported
93when a shared library is built, and in these cases the undocumented symbols are
94not exported.
95</P>
96<br><a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a><br>
97<P>
98If you are using PCRE2 in a non-UTF application that permits users to supply
99arbitrary patterns for compilation, you should be aware of a feature that
100allows users to turn on UTF support from within a pattern. For example, an
1018-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets
102patterns and subjects as strings of UTF-8 code units instead of individual
1038-bit characters. This causes both the pattern and any data against which it is
104matched to be checked for UTF-8 validity. If the data string is very long, such
105a check might use sufficiently many resources as to cause your application to
106lose performance.
107</P>
108<P>
109One way of guarding against this possibility is to use the
110<b>pcre2_pattern_info()</b> function to check the compiled pattern's options for
111PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling
112<b>pcre2_compile()</b>. This causes an compile time error if a pattern contains
113a UTF-setting sequence.
114</P>
115<P>
116The use of Unicode properties for character types such as \d can also be
117enabled from within the pattern, by specifying "(*UCP)". This feature can be
118disallowed by setting the PCRE2_NEVER_UCP option.
119</P>
120<P>
121If your application is one that supports UTF, be aware that validity checking
122can take time. If the same data string is to be matched many times, you can use
123the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid
124running redundant checks.
125</P>
126<P>
127The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to
128problems, because it may leave the current matching point in the middle of a
129multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an
130application to lock out the use of \C, causing a compile-time error if it is
131encountered. It is also possible to build PCRE2 with the use of \C permanently
132disabled.
133</P>
134<P>
135Another way that performance can be hit is by running a pattern that has a very
136large search tree against a string that will never match. Nested unlimited
137repeats in a pattern are a common example. PCRE2 provides some protection
138against this: see the <b>pcre2_set_match_limit()</b> function in the
139<a href="pcre2api.html"><b>pcre2api</b></a>
140page.
141</P>
142<br><a name="SEC3" href="#TOC1">USER DOCUMENTATION</a><br>
143<P>
144The user documentation for PCRE2 comprises a number of different sections. In
145the "man" format, each of these is a separate "man page". In the HTML format,
146each is a separate page, linked from the index page. In the plain text format,
147the descriptions of the <b>pcre2grep</b> and <b>pcre2test</b> programs are in
148files called <b>pcre2grep.txt</b> and <b>pcre2test.txt</b>, respectively. The
149remaining sections, except for the <b>pcre2demo</b> section (which is a program
150listing), and the short pages for individual functions, are concatenated in
151<b>pcre2.txt</b>, for ease of searching. The sections are as follows:
152<pre>
153  pcre2              this document
154  pcre2-config       show PCRE2 installation configuration information
155  pcre2api           details of PCRE2's native C API
156  pcre2build         building PCRE2
157  pcre2callout       details of the callout feature
158  pcre2compat        discussion of Perl compatibility
159  pcre2demo          a demonstration C program that uses PCRE2
160  pcre2grep          description of the <b>pcre2grep</b> command (8-bit only)
161  pcre2jit           discussion of just-in-time optimization support
162  pcre2limits        details of size and other limits
163  pcre2matching      discussion of the two matching algorithms
164  pcre2partial       details of the partial matching facility
165  pcre2pattern       syntax and semantics of supported regular expression patterns
166  pcre2perform       discussion of performance issues
167  pcre2posix         the POSIX-compatible C API for the 8-bit library
168  pcre2sample        discussion of the pcre2demo program
169  pcre2stack         discussion of stack usage
170  pcre2syntax        quick syntax reference
171  pcre2test          description of the <b>pcre2test</b> command
172  pcre2unicode       discussion of Unicode and UTF support
173</pre>
174In the "man" and HTML formats, there is also a short page for each C library
175function, listing its arguments and results.
176</P>
177<br><a name="SEC4" href="#TOC1">AUTHOR</a><br>
178<P>
179Philip Hazel
180<br>
181University Computing Service
182<br>
183Cambridge, England.
184<br>
185</P>
186<P>
187Putting an actual email address here is a spam magnet. If you want to email me,
188use my two initials, followed by the two digits 10, at the domain cam.ac.uk.
189</P>
190<br><a name="SEC5" href="#TOC1">REVISION</a><br>
191<P>
192Last updated: 16 October 2015
193<br>
194Copyright &copy; 1997-2015 University of Cambridge.
195<br>
196<p>
197Return to the <a href="index.html">PCRE2 index page</a>.
198</p>
199