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> 8 Return to the <a href="index.html">PCRE2 index page</a>. 9 </p> 10 <p> 11 This page is part of the PCRE2 HTML documentation. It was generated 12 automatically from the original man page. If there is any nonsense in it, 13 please 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> 24 PCRE2 is the name used for a revised API for the PCRE library, which is a set 25 of functions, written in C, that implement regular expression pattern matching 26 using the same syntax and semantics as Perl, with just a few differences. Some 27 features that appeared in Python and the original PCRE before they appeared in 28 Perl are also available using the Python syntax. There is also some support for 29 one or two .NET and Oniguruma syntax items, and there are options for 30 requesting some minor changes that give better ECMAScript (aka JavaScript) 31 compatibility. 32 </P> 33 <P> 34 The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit 35 code units, which means that up to three separate libraries may be installed. 36 The original work to extend PCRE to 16-bit and 32-bit code units was done by 37 Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings 38 can be interpreted either as one character per code unit, or as UTF-encoded 39 Unicode, with support for Unicode general category properties. Unicode support 40 is optional at build time (but is the default). However, processing strings as 41 UTF code units must be enabled explicitly at run time. The version of Unicode 42 in use can be discovered by running 43 <pre> 44 pcre2test -C 45 </PRE> 46 </P> 47 <P> 48 The 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, 50 by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just 51 one 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 53 the case. 54 </P> 55 <P> 56 In addition to the Perl-compatible matching function, PCRE2 contains an 57 alternative function that matches the same compiled patterns in a different 58 way. In certain circumstances, the alternative function has some advantages. 59 For a discussion of the two matching algorithms, see the 60 <a href="pcre2matching.html"><b>pcre2matching</b></a> 61 page. 62 </P> 63 <P> 64 Details of exactly which Perl regular expression features are and are not 65 supported by PCRE2 are given in separate documents. See the 66 <a href="pcre2pattern.html"><b>pcre2pattern</b></a> 67 and 68 <a href="pcre2compat.html"><b>pcre2compat</b></a> 69 pages. There is a syntax summary in the 70 <a href="pcre2syntax.html"><b>pcre2syntax</b></a> 71 page. 72 </P> 73 <P> 74 Some features of PCRE2 can be included, excluded, or changed when the library 75 is built. The 76 <a href="pcre2_config.html"><b>pcre2_config()</b></a> 77 function makes it possible for a client to discover which features are 78 available. The features themselves are described in the 79 <a href="pcre2build.html"><b>pcre2build</b></a> 80 page. Documentation about building PCRE2 for various operating systems can be 81 found in the 82 <a href="README.txt"><b>README</b></a> 83 and 84 <a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a> 85 files in the source distribution. 86 </P> 87 <P> 88 The libraries contains a number of undocumented internal functions and data 89 tables that are used by more than one of the exported external functions, but 90 which 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 92 environments, it is possible to control which external symbols are exported 93 when a shared library is built, and in these cases the undocumented symbols are 94 not exported. 95 </P> 96 <br><a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a><br> 97 <P> 98 If you are using PCRE2 in a non-UTF application that permits users to supply 99 arbitrary patterns for compilation, you should be aware of a feature that 100 allows users to turn on UTF support from within a pattern. For example, an 101 8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets 102 patterns and subjects as strings of UTF-8 code units instead of individual 103 8-bit characters. This causes both the pattern and any data against which it is 104 matched to be checked for UTF-8 validity. If the data string is very long, such 105 a check might use sufficiently many resources as to cause your application to 106 lose performance. 107 </P> 108 <P> 109 One 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 111 PCRE2_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 113 a UTF-setting sequence. 114 </P> 115 <P> 116 The use of Unicode properties for character types such as \d can also be 117 enabled from within the pattern, by specifying "(*UCP)". This feature can be 118 disallowed by setting the PCRE2_NEVER_UCP option. 119 </P> 120 <P> 121 If your application is one that supports UTF, be aware that validity checking 122 can take time. If the same data string is to be matched many times, you can use 123 the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid 124 running redundant checks. 125 </P> 126 <P> 127 The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to 128 problems, because it may leave the current matching point in the middle of a 129 multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an 130 application to lock out the use of \C, causing a compile-time error if it is 131 encountered. It is also possible to build PCRE2 with the use of \C permanently 132 disabled. 133 </P> 134 <P> 135 Another way that performance can be hit is by running a pattern that has a very 136 large search tree against a string that will never match. Nested unlimited 137 repeats in a pattern are a common example. PCRE2 provides some protection 138 against this: see the <b>pcre2_set_match_limit()</b> function in the 139 <a href="pcre2api.html"><b>pcre2api</b></a> 140 page. 141 </P> 142 <br><a name="SEC3" href="#TOC1">USER DOCUMENTATION</a><br> 143 <P> 144 The user documentation for PCRE2 comprises a number of different sections. In 145 the "man" format, each of these is a separate "man page". In the HTML format, 146 each is a separate page, linked from the index page. In the plain text format, 147 the descriptions of the <b>pcre2grep</b> and <b>pcre2test</b> programs are in 148 files called <b>pcre2grep.txt</b> and <b>pcre2test.txt</b>, respectively. The 149 remaining sections, except for the <b>pcre2demo</b> section (which is a program 150 listing), 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> 174 In the "man" and HTML formats, there is also a short page for each C library 175 function, listing its arguments and results. 176 </P> 177 <br><a name="SEC4" href="#TOC1">AUTHOR</a><br> 178 <P> 179 Philip Hazel 180 <br> 181 University Computing Service 182 <br> 183 Cambridge, England. 184 <br> 185 </P> 186 <P> 187 Putting an actual email address here is a spam magnet. If you want to email me, 188 use 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> 192 Last updated: 16 October 2015 193 <br> 194 Copyright © 1997-2015 University of Cambridge. 195 <br> 196 <p> 197 Return to the <a href="index.html">PCRE2 index page</a>. 198 </p> 199
