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1 <html>
2 <head>
3 <title>pcre specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcre man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 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">USER DOCUMENTATION</a>
18 <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 </ul>
22 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23 <P>
24 The PCRE library is a set of functions that implement regular expression
25 pattern matching using the same syntax and semantics as Perl, with just a few
26 differences. The current implementation of PCRE (release 6.x) corresponds
27 approximately with Perl 5.8, including support for UTF-8 encoded strings and
28 Unicode general category properties. However, this support has to be explicitly
29 enabled; it is not the default.
30 </P>
31 <P>
32 In addition to the Perl-compatible matching function, PCRE also contains an
33 alternative matching function that matches the same compiled patterns in a
34 different way. In certain circumstances, the alternative function has some
35 advantages. For a discussion of the two matching algorithms, see the
36 <a href="pcrematching.html"><b>pcrematching</b></a>
37 page.
38 </P>
39 <P>
40 PCRE is written in C and released as a C library. A number of people have
41 written wrappers and interfaces of various kinds. In particular, Google Inc.
42 have provided a comprehensive C++ wrapper. This is now included as part of the
43 PCRE distribution. The
44 <a href="pcrecpp.html"><b>pcrecpp</b></a>
45 page has details of this interface. Other people's contributions can be found
46 in the <i>Contrib</i> directory at the primary FTP site, which is:
47 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
48 </P>
49 <P>
50 Details of exactly which Perl regular expression features are and are not
51 supported by PCRE are given in separate documents. See the
52 <a href="pcrepattern.html"><b>pcrepattern</b></a>
53 and
54 <a href="pcrecompat.html"><b>pcrecompat</b></a>
55 pages.
56 </P>
57 <P>
58 Some features of PCRE can be included, excluded, or changed when the library is
59 built. The
60 <a href="pcre_config.html"><b>pcre_config()</b></a>
61 function makes it possible for a client to discover which features are
62 available. The features themselves are described in the
63 <a href="pcrebuild.html"><b>pcrebuild</b></a>
64 page. Documentation about building PCRE for various operating systems can be
65 found in the <b>README</b> file in the source distribution.
66 </P>
67 <P>
68 The library contains a number of undocumented internal functions and data
69 tables that are used by more than one of the exported external functions, but
70 which are not intended for use by external callers. Their names all begin with
71 "_pcre_", which hopefully will not provoke any name clashes. In some
72 environments, it is possible to control which external symbols are exported
73 when a shared library is built, and in these cases the undocumented symbols are
74 not exported.
75 </P>
76 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
77 <P>
78 The user documentation for PCRE comprises a number of different sections. In
79 the "man" format, each of these is a separate "man page". In the HTML format,
80 each is a separate page, linked from the index page. In the plain text format,
81 all the sections are concatenated, for ease of searching. The sections are as
82 follows:
83 <pre>
84 pcre this document
85 pcreapi details of PCRE's native C API
86 pcrebuild options for building PCRE
87 pcrecallout details of the callout feature
88 pcrecompat discussion of Perl compatibility
89 pcrecpp details of the C++ wrapper
90 pcregrep description of the <b>pcregrep</b> command
91 pcrematching discussion of the two matching algorithms
92 pcrepartial details of the partial matching facility
93 pcrepattern syntax and semantics of supported regular expressions
94 pcreperform discussion of performance issues
95 pcreposix the POSIX-compatible C API
96 pcreprecompile details of saving and re-using precompiled patterns
97 pcresample discussion of the sample program
98 pcrestack discussion of stack usage
99 pcretest description of the <b>pcretest</b> testing command
100 </pre>
101 In addition, in the "man" and HTML formats, there is a short page for each
102 C library function, listing its arguments and results.
103 </P>
104 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
105 <P>
106 There are some size limitations in PCRE but it is hoped that they will never in
107 practice be relevant.
108 </P>
109 <P>
110 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
111 compiled with the default internal linkage size of 2. If you want to process
112 regular expressions that are truly enormous, you can compile PCRE with an
113 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
114 distribution and the
115 <a href="pcrebuild.html"><b>pcrebuild</b></a>
116 documentation for details). In these cases the limit is substantially larger.
117 However, the speed of execution will be slower.
118 </P>
119 <P>
120 All values in repeating quantifiers must be less than 65536. The maximum
121 compiled length of subpattern with an explicit repeat count is 30000 bytes. The
122 maximum number of capturing subpatterns is 65535.
123 </P>
124 <P>
125 There is no limit to the number of non-capturing subpatterns, but the maximum
126 depth of nesting of all kinds of parenthesized subpattern, including capturing
127 subpatterns, assertions, and other types of subpattern, is 200.
128 </P>
129 <P>
130 The maximum length of name for a named subpattern is 32, and the maximum number
131 of named subpatterns is 10000.
132 </P>
133 <P>
134 The maximum length of a subject string is the largest positive number that an
135 integer variable can hold. However, when using the traditional matching
136 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
137 This means that the available stack space may limit the size of a subject
138 string that can be processed by certain patterns. For a discussion of stack
139 issues, see the
140 <a href="pcrestack.html"><b>pcrestack</b></a>
141 documentation.
142 <a name="utf8support"></a></P>
143 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
144 <P>
145 From release 3.3, PCRE has had some support for character strings encoded in
146 the UTF-8 format. For release 4.0 this was greatly extended to cover most
147 common requirements, and in release 5.0 additional support for Unicode general
148 category properties was added.
149 </P>
150 <P>
151 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
152 the code, and, in addition, you must call
153 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
154 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
155 subject strings that are matched against it are treated as UTF-8 strings
156 instead of just strings of bytes.
157 </P>
158 <P>
159 If you compile PCRE with UTF-8 support, but do not use it at run time, the
160 library will be a bit bigger, but the additional run time overhead is limited
161 to testing the PCRE_UTF8 flag in several places, so should not be very large.
162 </P>
163 <P>
164 If PCRE is built with Unicode character property support (which implies UTF-8
165 support), the escape sequences \p{..}, \P{..}, and \X are supported.
166 The available properties that can be tested are limited to the general
167 category properties such as Lu for an upper case letter or Nd for a decimal
168 number, the Unicode script names such as Arabic or Han, and the derived
169 properties Any and L&. A full list is given in the
170 <a href="pcrepattern.html"><b>pcrepattern</b></a>
171 documentation. Only the short names for properties are supported. For example,
172 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
173 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
174 compatibility with Perl 5.6. PCRE does not support this.
175 </P>
176 <P>
177 The following comments apply when PCRE is running in UTF-8 mode:
178 </P>
179 <P>
180 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
181 are checked for validity on entry to the relevant functions. If an invalid
182 UTF-8 string is passed, an error return is given. In some situations, you may
183 already know that your strings are valid, and therefore want to skip these
184 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
185 at compile time or at run time, PCRE assumes that the pattern or subject it
186 is given (respectively) contains only valid UTF-8 codes. In this case, it does
187 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
188 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
189 may crash.
190 </P>
191 <P>
192 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
193 UTF-8 character if the value is greater than 127.
194 </P>
195 <P>
196 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
197 characters for values greater than \177.
198 </P>
199 <P>
200 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
201 bytes, for example: \x{100}{3}.
202 </P>
203 <P>
204 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
205 </P>
206 <P>
207 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
208 but its use can lead to some strange effects. This facility is not available in
209 the alternative matching function, <b>pcre_dfa_exec()</b>.
210 </P>
211 <P>
212 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
213 test characters of any code value, but the characters that PCRE recognizes as
214 digits, spaces, or word characters remain the same set as before, all with
215 values less than 256. This remains true even when PCRE includes Unicode
216 property support, because to do otherwise would slow down PCRE in many common
217 cases. If you really want to test for a wider sense of, say, "digit", you
218 must use Unicode property tests such as \p{Nd}.
219 </P>
220 <P>
221 8. Similarly, characters that match the POSIX named character classes are all
222 low-valued characters.
223 </P>
224 <P>
225 9. Case-insensitive matching applies only to characters whose values are less
226 than 128, unless PCRE is built with Unicode property support. Even when Unicode
227 property support is available, PCRE still uses its own character tables when
228 checking the case of low-valued characters, so as not to degrade performance.
229 The Unicode property information is used only for characters with higher
230 values. Even when Unicode property support is available, PCRE supports
231 case-insensitive matching only when there is a one-to-one mapping between a
232 letter's cases. There are a small number of many-to-one mappings in Unicode;
233 these are not supported by PCRE.
234 </P>
235 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
236 <P>
237 Philip Hazel
238 <br>
239 University Computing Service,
240 <br>
241 Cambridge CB2 3QG, England.
242 </P>
243 <P>
244 Putting an actual email address here seems to have been a spam magnet, so I've
245 taken it away. If you want to email me, use my initial and surname, separated
246 by a dot, at the domain ucs.cam.ac.uk.
247 Last updated: 05 June 2006
248 <br>
249 Copyright &copy; 1997-2006 University of Cambridge.
250 <p>
251 Return to the <a href="index.html">PCRE index page</a>.
252 </p>

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