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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 pcretest description of the <b>pcretest</b> testing command
99 </pre>
100 In addition, in the "man" and HTML formats, there is a short page for each
101 C library function, listing its arguments and results.
102 </P>
103 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
104 <P>
105 There are some size limitations in PCRE but it is hoped that they will never in
106 practice be relevant.
107 </P>
108 <P>
109 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
110 compiled with the default internal linkage size of 2. If you want to process
111 regular expressions that are truly enormous, you can compile PCRE with an
112 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
113 distribution and the
114 <a href="pcrebuild.html"><b>pcrebuild</b></a>
115 documentation for details). In these cases the limit is substantially larger.
116 However, the speed of execution will be slower.
117 </P>
118 <P>
119 All values in repeating quantifiers must be less than 65536.
120 The maximum number of capturing subpatterns is 65535.
121 </P>
122 <P>
123 There is no limit to the number of non-capturing subpatterns, but the maximum
124 depth of nesting of all kinds of parenthesized subpattern, including capturing
125 subpatterns, assertions, and other types of subpattern, is 200.
126 </P>
127 <P>
128 The maximum length of a subject string is the largest positive number that an
129 integer variable can hold. However, when using the traditional matching
130 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
131 This means that the available stack space may limit the size of a subject
132 string that can be processed by certain patterns.
133 <a name="utf8support"></a></P>
134 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
135 <P>
136 From release 3.3, PCRE has had some support for character strings encoded in
137 the UTF-8 format. For release 4.0 this was greatly extended to cover most
138 common requirements, and in release 5.0 additional support for Unicode general
139 category properties was added.
140 </P>
141 <P>
142 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
143 the code, and, in addition, you must call
144 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
145 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
146 subject strings that are matched against it are treated as UTF-8 strings
147 instead of just strings of bytes.
148 </P>
149 <P>
150 If you compile PCRE with UTF-8 support, but do not use it at run time, the
151 library will be a bit bigger, but the additional run time overhead is limited
152 to testing the PCRE_UTF8 flag in several places, so should not be very large.
153 </P>
154 <P>
155 If PCRE is built with Unicode character property support (which implies UTF-8
156 support), the escape sequences \p{..}, \P{..}, and \X are supported.
157 The available properties that can be tested are limited to the general
158 category properties such as Lu for an upper case letter or Nd for a decimal
159 number. A full list is given in the
160 <a href="pcrepattern.html"><b>pcrepattern</b></a>
161 documentation. The PCRE library is increased in size by about 90K when Unicode
162 property support is included.
163 </P>
164 <P>
165 The following comments apply when PCRE is running in UTF-8 mode:
166 </P>
167 <P>
168 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
169 are checked for validity on entry to the relevant functions. If an invalid
170 UTF-8 string is passed, an error return is given. In some situations, you may
171 already know that your strings are valid, and therefore want to skip these
172 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
173 at compile time or at run time, PCRE assumes that the pattern or subject it
174 is given (respectively) contains only valid UTF-8 codes. In this case, it does
175 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
176 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
177 may crash.
178 </P>
179 <P>
180 2. In a pattern, the escape sequence \x{...}, where the contents of the braces
181 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
182 code number is the given hexadecimal number, for example: \x{1234}. If a
183 non-hexadecimal digit appears between the braces, the item is not recognized.
184 This escape sequence can be used either as a literal, or within a character
185 class.
186 </P>
187 <P>
188 3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
189 character if the value is greater than 127.
190 </P>
191 <P>
192 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
193 bytes, for example: \x{100}{3}.
194 </P>
195 <P>
196 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
197 </P>
198 <P>
199 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
200 but its use can lead to some strange effects. This facility is not available in
201 the alternative matching function, <b>pcre_dfa_exec()</b>.
202 </P>
203 <P>
204 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
205 test characters of any code value, but the characters that PCRE recognizes as
206 digits, spaces, or word characters remain the same set as before, all with
207 values less than 256. This remains true even when PCRE includes Unicode
208 property support, because to do otherwise would slow down PCRE in many common
209 cases. If you really want to test for a wider sense of, say, "digit", you
210 must use Unicode property tests such as \p{Nd}.
211 </P>
212 <P>
213 8. Similarly, characters that match the POSIX named character classes are all
214 low-valued characters.
215 </P>
216 <P>
217 9. Case-insensitive matching applies only to characters whose values are less
218 than 128, unless PCRE is built with Unicode property support. Even when Unicode
219 property support is available, PCRE still uses its own character tables when
220 checking the case of low-valued characters, so as not to degrade performance.
221 The Unicode property information is used only for characters with higher
222 values.
223 </P>
224 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
225 <P>
226 Philip Hazel
227 <br>
228 University Computing Service,
229 <br>
230 Cambridge CB2 3QG, England.
231 </P>
232 <P>
233 Putting an actual email address here seems to have been a spam magnet, so I've
234 taken it away. If you want to email me, use my initial and surname, separated
235 by a dot, at the domain ucs.cam.ac.uk.
236 Last updated: 07 March 2005
237 <br>
238 Copyright &copy; 1997-2005 University of Cambridge.
239 <p>
240 Return to the <a href="index.html">PCRE index page</a>.
241 </p>

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