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. After 27nearly two decades, the limitations of the original API were making development 28increasingly difficult. The new API is more extensible, and it was simplified 29by abolishing the separate "study" optimizing function; in PCRE2, patterns are 30automatically optimized where possible. Since forking from PCRE1, the code has 31been extensively refactored and new features introduced. 32</P> 33<P> 34As well as Perl-style regular expression patterns, some features that appeared 35in Python and the original PCRE before they appeared in Perl are available 36using the Python syntax. There is also some support for one or two .NET and 37Oniguruma syntax items, and there are options for requesting some minor changes 38that give better ECMAScript (aka JavaScript) compatibility. 39</P> 40<P> 41The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit 42code units, which means that up to three separate libraries may be installed. 43The original work to extend PCRE to 16-bit and 32-bit code units was done by 44Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings 45can be interpreted either as one character per code unit, or as UTF-encoded 46Unicode, with support for Unicode general category properties. Unicode support 47is optional at build time (but is the default). However, processing strings as 48UTF code units must be enabled explicitly at run time. The version of Unicode 49in use can be discovered by running 50<pre> 51 pcre2test -C 52</PRE> 53</P> 54<P> 55The three libraries contain identical sets of functions, with names ending in 56_8, _16, or _32, respectively (for example, <b>pcre2_compile_8()</b>). However, 57by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just 58one code unit width can be written using generic names such as 59<b>pcre2_compile()</b>, and the documentation is written assuming that this is 60the case. 61</P> 62<P> 63In addition to the Perl-compatible matching function, PCRE2 contains an 64alternative function that matches the same compiled patterns in a different 65way. In certain circumstances, the alternative function has some advantages. 66For a discussion of the two matching algorithms, see the 67<a href="pcre2matching.html"><b>pcre2matching</b></a> 68page. 69</P> 70<P> 71Details of exactly which Perl regular expression features are and are not 72supported by PCRE2 are given in separate documents. See the 73<a href="pcre2pattern.html"><b>pcre2pattern</b></a> 74and 75<a href="pcre2compat.html"><b>pcre2compat</b></a> 76pages. There is a syntax summary in the 77<a href="pcre2syntax.html"><b>pcre2syntax</b></a> 78page. 79</P> 80<P> 81Some features of PCRE2 can be included, excluded, or changed when the library 82is built. The 83<a href="pcre2_config.html"><b>pcre2_config()</b></a> 84function makes it possible for a client to discover which features are 85available. The features themselves are described in the 86<a href="pcre2build.html"><b>pcre2build</b></a> 87page. Documentation about building PCRE2 for various operating systems can be 88found in the 89<a href="README.txt"><b>README</b></a> 90and 91<a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a> 92files in the source distribution. 93</P> 94<P> 95The libraries contains a number of undocumented internal functions and data 96tables that are used by more than one of the exported external functions, but 97which are not intended for use by external callers. Their names all begin with 98"_pcre2", which hopefully will not provoke any name clashes. In some 99environments, it is possible to control which external symbols are exported 100when a shared library is built, and in these cases the undocumented symbols are 101not exported. 102</P> 103<br><a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a><br> 104<P> 105If you are using PCRE2 in a non-UTF application that permits users to supply 106arbitrary patterns for compilation, you should be aware of a feature that 107allows users to turn on UTF support from within a pattern. For example, an 1088-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets 109patterns and subjects as strings of UTF-8 code units instead of individual 1108-bit characters. This causes both the pattern and any data against which it is 111matched to be checked for UTF-8 validity. If the data string is very long, such 112a check might use sufficiently many resources as to cause your application to 113lose performance. 114</P> 115<P> 116One way of guarding against this possibility is to use the 117<b>pcre2_pattern_info()</b> function to check the compiled pattern's options for 118PCRE2_UTF. Alternatively, you can set the PCRE2_NEVER_UTF option when calling 119<b>pcre2_compile()</b>. This causes a compile time error if the pattern contains 120a UTF-setting sequence. 121</P> 122<P> 123The use of Unicode properties for character types such as \d can also be 124enabled from within the pattern, by specifying "(*UCP)". This feature can be 125disallowed by setting the PCRE2_NEVER_UCP option. 126</P> 127<P> 128If your application is one that supports UTF, be aware that validity checking 129can take time. If the same data string is to be matched many times, you can use 130the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid 131running redundant checks. 132</P> 133<P> 134The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to 135problems, because it may leave the current matching point in the middle of a 136multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an 137application to lock out the use of \C, causing a compile-time error if it is 138encountered. It is also possible to build PCRE2 with the use of \C permanently 139disabled. 140</P> 141<P> 142Another way that performance can be hit is by running a pattern that has a very 143large search tree against a string that will never match. Nested unlimited 144repeats in a pattern are a common example. PCRE2 provides some protection 145against this: see the <b>pcre2_set_match_limit()</b> function in the 146<a href="pcre2api.html"><b>pcre2api</b></a> 147page. There is a similar function called <b>pcre2_set_depth_limit()</b> that can 148be used to restrict the amount of memory that is used. 149</P> 150<br><a name="SEC3" href="#TOC1">USER DOCUMENTATION</a><br> 151<P> 152The user documentation for PCRE2 comprises a number of different sections. In 153the "man" format, each of these is a separate "man page". In the HTML format, 154each is a separate page, linked from the index page. In the plain text format, 155the descriptions of the <b>pcre2grep</b> and <b>pcre2test</b> programs are in 156files called <b>pcre2grep.txt</b> and <b>pcre2test.txt</b>, respectively. The 157remaining sections, except for the <b>pcre2demo</b> section (which is a program 158listing), and the short pages for individual functions, are concatenated in 159<b>pcre2.txt</b>, for ease of searching. The sections are as follows: 160<pre> 161 pcre2 this document 162 pcre2-config show PCRE2 installation configuration information 163 pcre2api details of PCRE2's native C API 164 pcre2build building PCRE2 165 pcre2callout details of the callout feature 166 pcre2compat discussion of Perl compatibility 167 pcre2convert details of pattern conversion functions 168 pcre2demo a demonstration C program that uses PCRE2 169 pcre2grep description of the <b>pcre2grep</b> command (8-bit only) 170 pcre2jit discussion of just-in-time optimization support 171 pcre2limits details of size and other limits 172 pcre2matching discussion of the two matching algorithms 173 pcre2partial details of the partial matching facility 174 pcre2pattern syntax and semantics of supported regular expression patterns 175 pcre2perform discussion of performance issues 176 pcre2posix the POSIX-compatible C API for the 8-bit library 177 pcre2sample discussion of the pcre2demo program 178 pcre2serialize details of pattern serialization 179 pcre2syntax quick syntax reference 180 pcre2test description of the <b>pcre2test</b> command 181 pcre2unicode discussion of Unicode and UTF support 182</pre> 183In the "man" and HTML formats, there is also a short page for each C library 184function, listing its arguments and results. 185</P> 186<br><a name="SEC4" href="#TOC1">AUTHOR</a><br> 187<P> 188Philip Hazel 189<br> 190University Computing Service 191<br> 192Cambridge, England. 193<br> 194</P> 195<P> 196Putting an actual email address here is a spam magnet. If you want to email me, 197use my two initials, followed by the two digits 10, at the domain cam.ac.uk. 198</P> 199<br><a name="SEC5" href="#TOC1">REVISION</a><br> 200<P> 201Last updated: 11 July 2018 202<br> 203Copyright © 1997-2018 University of Cambridge. 204<br> 205<p> 206Return to the <a href="index.html">PCRE2 index page</a>. 207</p> 208