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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 5.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 PCRE is written in C and released as a C library. A number of people have
33 written wrappers and interfaces of various kinds. A C++ class is included in
34 these contributions, which can be found in the <i>Contrib</i> directory at the
35 primary FTP site, which is:
36 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
37 </P>
38 <P>
39 Details of exactly which Perl regular expression features are and are not
40 supported by PCRE are given in separate documents. See the
41 <a href="pcrepattern.html"><b>pcrepattern</b></a>
42 and
43 <a href="pcrecompat.html"><b>pcrecompat</b></a>
44 pages.
45 </P>
46 <P>
47 Some features of PCRE can be included, excluded, or changed when the library is
48 built. The
49 <a href="pcre_config.html"><b>pcre_config()</b></a>
50 function makes it possible for a client to discover which features are
51 available. The features themselves are described in the
52 <a href="pcrebuild.html"><b>pcrebuild</b></a>
53 page. Documentation about building PCRE for various operating systems can be
54 found in the <b>README</b> file in the source distribution.
55 </P>
56 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
57 <P>
58 The user documentation for PCRE comprises a number of different sections. In
59 the "man" format, each of these is a separate "man page". In the HTML format,
60 each is a separate page, linked from the index page. In the plain text format,
61 all the sections are concatenated, for ease of searching. The sections are as
62 follows:
63 <pre>
64 pcre this document
65 pcreapi details of PCRE's native API
66 pcrebuild options for building PCRE
67 pcrecallout details of the callout feature
68 pcrecompat discussion of Perl compatibility
69 pcregrep description of the <b>pcregrep</b> command
70 pcrepartial details of the partial matching facility
71 pcrepattern syntax and semantics of supported regular expressions
72 pcreperform discussion of performance issues
73 pcreposix the POSIX-compatible API
74 pcreprecompile details of saving and re-using precompiled patterns
75 pcresample discussion of the sample program
76 pcretest description of the <b>pcretest</b> testing command
77 </pre>
78 In addition, in the "man" and HTML formats, there is a short page for each
79 library function, listing its arguments and results.
80 </P>
81 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
82 <P>
83 There are some size limitations in PCRE but it is hoped that they will never in
84 practice be relevant.
85 </P>
86 <P>
87 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
88 compiled with the default internal linkage size of 2. If you want to process
89 regular expressions that are truly enormous, you can compile PCRE with an
90 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
91 distribution and the
92 <a href="pcrebuild.html"><b>pcrebuild</b></a>
93 documentation for details). In these cases the limit is substantially larger.
94 However, the speed of execution will be slower.
95 </P>
96 <P>
97 All values in repeating quantifiers must be less than 65536.
98 The maximum number of capturing subpatterns is 65535.
99 </P>
100 <P>
101 There is no limit to the number of non-capturing subpatterns, but the maximum
102 depth of nesting of all kinds of parenthesized subpattern, including capturing
103 subpatterns, assertions, and other types of subpattern, is 200.
104 </P>
105 <P>
106 The maximum length of a subject string is the largest positive number that an
107 integer variable can hold. However, PCRE uses recursion to handle subpatterns
108 and indefinite repetition. This means that the available stack space may limit
109 the size of a subject string that can be processed by certain patterns.
110 <a name="utf8support"></a></P>
111 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
112 <P>
113 From release 3.3, PCRE has had some support for character strings encoded in
114 the UTF-8 format. For release 4.0 this was greatly extended to cover most
115 common requirements, and in release 5.0 additional support for Unicode general
116 category properties was added.
117 </P>
118 <P>
119 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
120 the code, and, in addition, you must call
121 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
122 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
123 subject strings that are matched against it are treated as UTF-8 strings
124 instead of just strings of bytes.
125 </P>
126 <P>
127 If you compile PCRE with UTF-8 support, but do not use it at run time, the
128 library will be a bit bigger, but the additional run time overhead is limited
129 to testing the PCRE_UTF8 flag in several places, so should not be very large.
130 </P>
131 <P>
132 If PCRE is built with Unicode character property support (which implies UTF-8
133 support), the escape sequences \p{..}, \P{..}, and \X are supported.
134 The available properties that can be tested are limited to the general
135 category properties such as Lu for an upper case letter or Nd for a decimal
136 number. A full list is given in the
137 <a href="pcrepattern.html"><b>pcrepattern</b></a>
138 documentation. The PCRE library is increased in size by about 90K when Unicode
139 property support is included.
140 </P>
141 <P>
142 The following comments apply when PCRE is running in UTF-8 mode:
143 </P>
144 <P>
145 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
146 are checked for validity on entry to the relevant functions. If an invalid
147 UTF-8 string is passed, an error return is given. In some situations, you may
148 already know that your strings are valid, and therefore want to skip these
149 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
150 at compile time or at run time, PCRE assumes that the pattern or subject it
151 is given (respectively) contains only valid UTF-8 codes. In this case, it does
152 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
153 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
154 may crash.
155 </P>
156 <P>
157 2. In a pattern, the escape sequence \x{...}, where the contents of the braces
158 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
159 code number is the given hexadecimal number, for example: \x{1234}. If a
160 non-hexadecimal digit appears between the braces, the item is not recognized.
161 This escape sequence can be used either as a literal, or within a character
162 class.
163 </P>
164 <P>
165 3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
166 character if the value is greater than 127.
167 </P>
168 <P>
169 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
170 bytes, for example: \x{100}{3}.
171 </P>
172 <P>
173 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
174 </P>
175 <P>
176 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
177 but its use can lead to some strange effects.
178 </P>
179 <P>
180 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
181 test characters of any code value, but the characters that PCRE recognizes as
182 digits, spaces, or word characters remain the same set as before, all with
183 values less than 256. This remains true even when PCRE includes Unicode
184 property support, because to do otherwise would slow down PCRE in many common
185 cases. If you really want to test for a wider sense of, say, "digit", you
186 must use Unicode property tests such as \p{Nd}.
187 </P>
188 <P>
189 8. Similarly, characters that match the POSIX named character classes are all
190 low-valued characters.
191 </P>
192 <P>
193 9. Case-insensitive matching applies only to characters whose values are less
194 than 128, unless PCRE is built with Unicode property support. Even when Unicode
195 property support is available, PCRE still uses its own character tables when
196 checking the case of low-valued characters, so as not to degrade performance.
197 The Unicode property information is used only for characters with higher
198 values.
199 </P>
200 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
201 <P>
202 Philip Hazel &#60;ph10@cam.ac.uk&#62;
203 <br>
204 University Computing Service,
205 <br>
206 Cambridge CB2 3QG, England.
207 <br>
208 Phone: +44 1223 334714
209 Last updated: 09 September 2004
210 <br>
211 Copyright &copy; 1997-2004 University of Cambridge.
212 <p>
213 Return to the <a href="index.html">PCRE index page</a>.
214 </p>

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