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. After 27 nearly two decades, the limitations of the original API were making development 28 increasingly difficult. The new API is more extensible, and it was simplified 29 by abolishing the separate "study" optimizing function; in PCRE2, patterns are 30 automatically optimized where possible. Since forking from PCRE1, the code has 31 been extensively refactored and new features introduced. 32 </P> 33 <P> 34 As well as Perl-style regular expression patterns, some features that appeared 35 in Python and the original PCRE before they appeared in Perl are available 36 using the Python syntax. There is also some support for one or two .NET and 37 Oniguruma syntax items, and there are options for requesting some minor changes 38 that give better ECMAScript (aka JavaScript) compatibility. 39 </P> 40 <P> 41 The source code for PCRE2 can be compiled to support 8-bit, 16-bit, or 32-bit 42 code units, which means that up to three separate libraries may be installed. 43 The original work to extend PCRE to 16-bit and 32-bit code units was done by 44 Zoltan Herczeg and Christian Persch, respectively. In all three cases, strings 45 can be interpreted either as one character per code unit, or as UTF-encoded 46 Unicode, with support for Unicode general category properties. Unicode support 47 is optional at build time (but is the default). However, processing strings as 48 UTF code units must be enabled explicitly at run time. The version of Unicode 49 in use can be discovered by running 50 <pre> 51 pcre2test -C 52 </PRE> 53 </P> 54 <P> 55 The 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, 57 by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or 32, a program that uses just 58 one 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 60 the case. 61 </P> 62 <P> 63 In addition to the Perl-compatible matching function, PCRE2 contains an 64 alternative function that matches the same compiled patterns in a different 65 way. In certain circumstances, the alternative function has some advantages. 66 For a discussion of the two matching algorithms, see the 67 <a href="pcre2matching.html"><b>pcre2matching</b></a> 68 page. 69 </P> 70 <P> 71 Details of exactly which Perl regular expression features are and are not 72 supported by PCRE2 are given in separate documents. See the 73 <a href="pcre2pattern.html"><b>pcre2pattern</b></a> 74 and 75 <a href="pcre2compat.html"><b>pcre2compat</b></a> 76 pages. There is a syntax summary in the 77 <a href="pcre2syntax.html"><b>pcre2syntax</b></a> 78 page. 79 </P> 80 <P> 81 Some features of PCRE2 can be included, excluded, or changed when the library 82 is built. The 83 <a href="pcre2_config.html"><b>pcre2_config()</b></a> 84 function makes it possible for a client to discover which features are 85 available. The features themselves are described in the 86 <a href="pcre2build.html"><b>pcre2build</b></a> 87 page. Documentation about building PCRE2 for various operating systems can be 88 found in the 89 <a href="README.txt"><b>README</b></a> 90 and 91 <a href="NON-AUTOTOOLS-BUILD.txt"><b>NON-AUTOTOOLS_BUILD</b></a> 92 files in the source distribution. 93 </P> 94 <P> 95 The libraries contains a number of undocumented internal functions and data 96 tables that are used by more than one of the exported external functions, but 97 which 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 99 environments, it is possible to control which external symbols are exported 100 when a shared library is built, and in these cases the undocumented symbols are 101 not exported. 102 </P> 103 <br><a name="SEC2" href="#TOC1">SECURITY CONSIDERATIONS</a><br> 104 <P> 105 If you are using PCRE2 in a non-UTF application that permits users to supply 106 arbitrary patterns for compilation, you should be aware of a feature that 107 allows users to turn on UTF support from within a pattern. For example, an 108 8-bit pattern that begins with "(*UTF)" turns on UTF-8 mode, which interprets 109 patterns and subjects as strings of UTF-8 code units instead of individual 110 8-bit characters. This causes both the pattern and any data against which it is 111 matched to be checked for UTF-8 validity. If the data string is very long, such 112 a check might use sufficiently many resources as to cause your application to 113 lose performance. 114 </P> 115 <P> 116 One 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 118 PCRE2_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 120 a UTF-setting sequence. 121 </P> 122 <P> 123 The use of Unicode properties for character types such as \d can also be 124 enabled from within the pattern, by specifying "(*UCP)". This feature can be 125 disallowed by setting the PCRE2_NEVER_UCP option. 126 </P> 127 <P> 128 If your application is one that supports UTF, be aware that validity checking 129 can take time. If the same data string is to be matched many times, you can use 130 the PCRE2_NO_UTF_CHECK option for the second and subsequent matches to avoid 131 running redundant checks. 132 </P> 133 <P> 134 The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead to 135 problems, because it may leave the current matching point in the middle of a 136 multi-code-unit character. The PCRE2_NEVER_BACKSLASH_C option can be used by an 137 application to lock out the use of \C, causing a compile-time error if it is 138 encountered. It is also possible to build PCRE2 with the use of \C permanently 139 disabled. 140 </P> 141 <P> 142 Another way that performance can be hit is by running a pattern that has a very 143 large search tree against a string that will never match. Nested unlimited 144 repeats in a pattern are a common example. PCRE2 provides some protection 145 against this: see the <b>pcre2_set_match_limit()</b> function in the 146 <a href="pcre2api.html"><b>pcre2api</b></a> 147 page. There is a similar function called <b>pcre2_set_depth_limit()</b> that can 148 be 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> 152 The user documentation for PCRE2 comprises a number of different sections. In 153 the "man" format, each of these is a separate "man page". In the HTML format, 154 each is a separate page, linked from the index page. In the plain text format, 155 the descriptions of the <b>pcre2grep</b> and <b>pcre2test</b> programs are in 156 files called <b>pcre2grep.txt</b> and <b>pcre2test.txt</b>, respectively. The 157 remaining sections, except for the <b>pcre2demo</b> section (which is a program 158 listing), 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> 183 In the "man" and HTML formats, there is also a short page for each C library 184 function, listing its arguments and results. 185 </P> 186 <br><a name="SEC4" href="#TOC1">AUTHOR</a><br> 187 <P> 188 Philip Hazel 189 <br> 190 University Computing Service 191 <br> 192 Cambridge, England. 193 <br> 194 </P> 195 <P> 196 Putting an actual email address here is a spam magnet. If you want to email me, 197 use 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> 201 Last updated: 11 July 2018 202 <br> 203 Copyright © 1997-2018 University of Cambridge. 204 <br> 205 <p> 206 Return to the <a href="index.html">PCRE2 index page</a>. 207 </p> 208
