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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 This HTML document has been generated automatically from the original man page.
7 If there is any nonsense in it, please consult the man page, in case the
8 conversion went wrong.<br>
9 <ul>
10 <li><a name="TOC1" href="#SEC1">DESCRIPTION</a>
11 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
12 <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
13 <li><a name="TOC4" href="#SEC4">UTF-8 SUPPORT</a>
14 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
15 </ul>
16 <br><a name="SEC1" href="#TOC1">DESCRIPTION</a><br>
17 <P>
18 The PCRE library is a set of functions that implement regular expression
19 pattern matching using the same syntax and semantics as Perl, with just a few
20 differences. The current implementation of PCRE (release 4.x) corresponds
21 approximately with Perl 5.8, including support for UTF-8 encoded strings.
22 However, this support has to be explicitly enabled; it is not the default.
23 </P>
24 <P>
25 PCRE is written in C and released as a C library. However, a number of people
26 have written wrappers and interfaces of various kinds. A C++ class is included
27 in these contributions, which can be found in the <i>Contrib</i> directory at
28 the primary FTP site, which is:
29 </P>
30 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
31 <P>
32 Details of exactly which Perl regular expression features are and are not
33 supported by PCRE are given in separate documents. See the
34 <a href="pcrepattern.html"><b>pcrepattern</b></a>
35 and
36 <a href="pcrecompat.html"><b>pcrecompat</b></a>
37 pages.
38 </P>
39 <P>
40 Some features of PCRE can be included, excluded, or changed when the library is
41 built. The
42 <a href="pcre_config.html"><b>pcre_config()</b></a>
43 function makes it possible for a client to discover which features are
44 available. Documentation about building PCRE for various operating systems can
45 be found in the <b>README</b> file in the source distribution.
46 </P>
47 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
48 <P>
49 The user documentation for PCRE has been split up into a number of different
50 sections. In the "man" format, each of these is a separate "man page". In the
51 HTML format, each is a separate page, linked from the index page. In the plain
52 text format, all the sections are concatenated, for ease of searching. The
53 sections are as follows:
54 </P>
55 <P>
56 <pre>
57 pcre this document
58 pcreapi details of PCRE's native API
59 pcrebuild options for building PCRE
60 pcrecallout details of the callout feature
61 pcrecompat discussion of Perl compatibility
62 pcregrep description of the <b>pcregrep</b> command
63 pcrepattern syntax and semantics of supported
64 regular expressions
65 pcreperform discussion of performance issues
66 pcreposix the POSIX-compatible API
67 pcresample discussion of the sample program
68 pcretest the <b>pcretest</b> testing command
69 </PRE>
70 </P>
71 <P>
72 In addition, in the "man" and HTML formats, there is a short page for each
73 library function, listing its arguments and results.
74 </P>
75 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
76 <P>
77 There are some size limitations in PCRE but it is hoped that they will never in
78 practice be relevant.
79 </P>
80 <P>
81 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
82 compiled with the default internal linkage size of 2. If you want to process
83 regular expressions that are truly enormous, you can compile PCRE with an
84 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
85 distribution and the
86 <a href="pcrebuild.html"><b>pcrebuild</b></a>
87 documentation for details). If these cases the limit is substantially larger.
88 However, the speed of execution will be slower.
89 </P>
90 <P>
91 All values in repeating quantifiers must be less than 65536.
92 The maximum number of capturing subpatterns is 65535.
93 </P>
94 <P>
95 There is no limit to the number of non-capturing subpatterns, but the maximum
96 depth of nesting of all kinds of parenthesized subpattern, including capturing
97 subpatterns, assertions, and other types of subpattern, is 200.
98 </P>
99 <P>
100 The maximum length of a subject string is the largest positive number that an
101 integer variable can hold. However, PCRE uses recursion to handle subpatterns
102 and indefinite repetition. This means that the available stack space may limit
103 the size of a subject string that can be processed by certain patterns.
104 </P>
105 <a name="utf8support"></a><br><a name="SEC4" href="#TOC1">UTF-8 SUPPORT</a><br>
106 <P>
107 Starting at release 3.3, PCRE has had some support for character strings
108 encoded in the UTF-8 format. For release 4.0 this has been greatly extended to
109 cover most common requirements.
110 </P>
111 <P>
112 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
113 the code, and, in addition, you must call
114 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
115 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
116 subject strings that are matched against it are treated as UTF-8 strings
117 instead of just strings of bytes.
118 </P>
119 <P>
120 If you compile PCRE with UTF-8 support, but do not use it at run time, the
121 library will be a bit bigger, but the additional run time overhead is limited
122 to testing the PCRE_UTF8 flag in several places, so should not be very large.
123 </P>
124 <P>
125 The following comments apply when PCRE is running in UTF-8 mode:
126 </P>
127 <P>
128 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
129 are checked for validity on entry to the relevant functions. If an invalid
130 UTF-8 string is passed, an error return is given. In some situations, you may
131 already know that your strings are valid, and therefore want to skip these
132 checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
133 at compile time or at run time, PCRE assumes that the pattern or subject it
134 is given (respectively) contains only valid UTF-8 codes. In this case, it does
135 not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
136 PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
137 may crash.
138 </P>
139 <P>
140 2. In a pattern, the escape sequence \x{...}, where the contents of the braces
141 is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
142 code number is the given hexadecimal number, for example: \x{1234}. If a
143 non-hexadecimal digit appears between the braces, the item is not recognized.
144 This escape sequence can be used either as a literal, or within a character
145 class.
146 </P>
147 <P>
148 3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
149 character if the value is greater than 127.
150 </P>
151 <P>
152 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
153 bytes, for example: \x{100}{3}.
154 </P>
155 <P>
156 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
157 </P>
158 <P>
159 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
160 but its use can lead to some strange effects.
161 </P>
162 <P>
163 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
164 test characters of any code value, but the characters that PCRE recognizes as
165 digits, spaces, or word characters remain the same set as before, all with
166 values less than 256.
167 </P>
168 <P>
169 8. Case-insensitive matching applies only to characters whose values are less
170 than 256. PCRE does not support the notion of "case" for higher-valued
171 characters.
172 </P>
173 <P>
174 9. PCRE does not support the use of Unicode tables and properties or the Perl
175 escapes \p, \P, and \X.
176 </P>
177 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
178 <P>
179 Philip Hazel &#60;ph10@cam.ac.uk&#62;
180 <br>
181 University Computing Service,
182 <br>
183 Cambridge CB2 3QG, England.
184 <br>
185 Phone: +44 1223 334714
186 </P>
187 <P>
188 Last updated: 20 August 2003
189 <br>
190 Copyright &copy; 1997-2003 University of Cambridge.

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