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1 nigel 63 <html>
2     <head>
3     <title>pcre specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <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 nigel 63 <ul>
16 nigel 75 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 nigel 63 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18     <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 nigel 75 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 nigel 63 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21     </ul>
22 nigel 75 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
23 nigel 63 <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 nigel 77 differences. The current implementation of PCRE (release 6.x) corresponds
27 nigel 75 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 nigel 63 </P>
31     <P>
32 nigel 77 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 nigel 75 PCRE is written in C and released as a C library. A number of people have
41 nigel 77 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 nigel 75 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
48 nigel 63 </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 nigel 75 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 nigel 63 </P>
67 nigel 77 <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 nigel 83 "_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 nigel 77 </P>
76 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
77     <P>
78 nigel 75 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 nigel 63 <pre>
84     pcre this document
85 nigel 77 pcreapi details of PCRE's native C API
86 nigel 63 pcrebuild options for building PCRE
87     pcrecallout details of the callout feature
88     pcrecompat discussion of Perl compatibility
89 nigel 77 pcrecpp details of the C++ wrapper
90 nigel 63 pcregrep description of the <b>pcregrep</b> command
91 nigel 77 pcrematching discussion of the two matching algorithms
92 nigel 75 pcrepartial details of the partial matching facility
93     pcrepattern syntax and semantics of supported regular expressions
94 nigel 63 pcreperform discussion of performance issues
95 nigel 77 pcreposix the POSIX-compatible C API
96 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
97 nigel 63 pcresample discussion of the sample program
98 nigel 75 pcretest description of the <b>pcretest</b> testing command
99     </pre>
100 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
101 nigel 77 C library function, listing its arguments and results.
102 nigel 63 </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 nigel 75 documentation for details). In these cases the limit is substantially larger.
116 nigel 63 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 nigel 77 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 nigel 75 <a name="utf8support"></a></P>
134     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
135 nigel 63 <P>
136 nigel 75 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 nigel 63 </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 nigel 75 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 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
166     </P>
167     <P>
168 nigel 71 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 nigel 63 </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 nigel 77 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 nigel 63 </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 nigel 75 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 nigel 63 </P>
212     <P>
213 nigel 75 8. Similarly, characters that match the POSIX named character classes are all
214     low-valued characters.
215 nigel 63 </P>
216     <P>
217 nigel 75 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 nigel 63 </P>
224     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
225     <P>
226 nigel 77 Philip Hazel
227 nigel 63 <br>
228     University Computing Service,
229     <br>
230     Cambridge CB2 3QG, England.
231 nigel 77 </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 nigel 63 <br>
238 nigel 77 Copyright &copy; 1997-2005 University of Cambridge.
239 nigel 75 <p>
240     Return to the <a href="index.html">PCRE index page</a>.
241     </p>

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