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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     "_pcre_", which hopefully will not provoke any name clashes.
72     </P>
73 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
74     <P>
75 nigel 75 The user documentation for PCRE comprises a number of different sections. In
76     the "man" format, each of these is a separate "man page". In the HTML format,
77     each is a separate page, linked from the index page. In the plain text format,
78     all the sections are concatenated, for ease of searching. The sections are as
79     follows:
80 nigel 63 <pre>
81     pcre this document
82 nigel 77 pcreapi details of PCRE's native C API
83 nigel 63 pcrebuild options for building PCRE
84     pcrecallout details of the callout feature
85     pcrecompat discussion of Perl compatibility
86 nigel 77 pcrecpp details of the C++ wrapper
87 nigel 63 pcregrep description of the <b>pcregrep</b> command
88 nigel 77 pcrematching discussion of the two matching algorithms
89 nigel 75 pcrepartial details of the partial matching facility
90     pcrepattern syntax and semantics of supported regular expressions
91 nigel 63 pcreperform discussion of performance issues
92 nigel 77 pcreposix the POSIX-compatible C API
93 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
94 nigel 63 pcresample discussion of the sample program
95 nigel 75 pcretest description of the <b>pcretest</b> testing command
96     </pre>
97 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
98 nigel 77 C library function, listing its arguments and results.
99 nigel 63 </P>
100     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
101     <P>
102     There are some size limitations in PCRE but it is hoped that they will never in
103     practice be relevant.
104     </P>
105     <P>
106     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
107     compiled with the default internal linkage size of 2. If you want to process
108     regular expressions that are truly enormous, you can compile PCRE with an
109     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
110     distribution and the
111     <a href="pcrebuild.html"><b>pcrebuild</b></a>
112 nigel 75 documentation for details). In these cases the limit is substantially larger.
113 nigel 63 However, the speed of execution will be slower.
114     </P>
115     <P>
116     All values in repeating quantifiers must be less than 65536.
117     The maximum number of capturing subpatterns is 65535.
118     </P>
119     <P>
120     There is no limit to the number of non-capturing subpatterns, but the maximum
121     depth of nesting of all kinds of parenthesized subpattern, including capturing
122     subpatterns, assertions, and other types of subpattern, is 200.
123     </P>
124     <P>
125     The maximum length of a subject string is the largest positive number that an
126 nigel 77 integer variable can hold. However, when using the traditional matching
127     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
128     This means that the available stack space may limit the size of a subject
129     string that can be processed by certain patterns.
130 nigel 75 <a name="utf8support"></a></P>
131     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
132 nigel 63 <P>
133 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
134     the UTF-8 format. For release 4.0 this was greatly extended to cover most
135     common requirements, and in release 5.0 additional support for Unicode general
136     category properties was added.
137 nigel 63 </P>
138     <P>
139     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
140     the code, and, in addition, you must call
141     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
142     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
143     subject strings that are matched against it are treated as UTF-8 strings
144     instead of just strings of bytes.
145     </P>
146     <P>
147     If you compile PCRE with UTF-8 support, but do not use it at run time, the
148     library will be a bit bigger, but the additional run time overhead is limited
149     to testing the PCRE_UTF8 flag in several places, so should not be very large.
150     </P>
151     <P>
152 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
153     support), the escape sequences \p{..}, \P{..}, and \X are supported.
154     The available properties that can be tested are limited to the general
155     category properties such as Lu for an upper case letter or Nd for a decimal
156     number. A full list is given in the
157     <a href="pcrepattern.html"><b>pcrepattern</b></a>
158     documentation. The PCRE library is increased in size by about 90K when Unicode
159     property support is included.
160     </P>
161     <P>
162 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
163     </P>
164     <P>
165 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
166     are checked for validity on entry to the relevant functions. If an invalid
167     UTF-8 string is passed, an error return is given. In some situations, you may
168     already know that your strings are valid, and therefore want to skip these
169     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
170     at compile time or at run time, PCRE assumes that the pattern or subject it
171     is given (respectively) contains only valid UTF-8 codes. In this case, it does
172     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
173     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
174     may crash.
175 nigel 63 </P>
176     <P>
177     2. In a pattern, the escape sequence \x{...}, where the contents of the braces
178     is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
179     code number is the given hexadecimal number, for example: \x{1234}. If a
180     non-hexadecimal digit appears between the braces, the item is not recognized.
181     This escape sequence can be used either as a literal, or within a character
182     class.
183     </P>
184     <P>
185     3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
186     character if the value is greater than 127.
187     </P>
188     <P>
189     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
190     bytes, for example: \x{100}{3}.
191     </P>
192     <P>
193     5. The dot metacharacter matches one UTF-8 character instead of a single byte.
194     </P>
195     <P>
196     6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
197 nigel 77 but its use can lead to some strange effects. This facility is not available in
198     the alternative matching function, <b>pcre_dfa_exec()</b>.
199 nigel 63 </P>
200     <P>
201     7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
202     test characters of any code value, but the characters that PCRE recognizes as
203     digits, spaces, or word characters remain the same set as before, all with
204 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
205     property support, because to do otherwise would slow down PCRE in many common
206     cases. If you really want to test for a wider sense of, say, "digit", you
207     must use Unicode property tests such as \p{Nd}.
208 nigel 63 </P>
209     <P>
210 nigel 75 8. Similarly, characters that match the POSIX named character classes are all
211     low-valued characters.
212 nigel 63 </P>
213     <P>
214 nigel 75 9. Case-insensitive matching applies only to characters whose values are less
215     than 128, unless PCRE is built with Unicode property support. Even when Unicode
216     property support is available, PCRE still uses its own character tables when
217     checking the case of low-valued characters, so as not to degrade performance.
218     The Unicode property information is used only for characters with higher
219     values.
220 nigel 63 </P>
221     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
222     <P>
223 nigel 77 Philip Hazel
224 nigel 63 <br>
225     University Computing Service,
226     <br>
227     Cambridge CB2 3QG, England.
228 nigel 77 </P>
229     <P>
230     Putting an actual email address here seems to have been a spam magnet, so I've
231     taken it away. If you want to email me, use my initial and surname, separated
232     by a dot, at the domain ucs.cam.ac.uk.
233     Last updated: 07 March 2005
234 nigel 63 <br>
235 nigel 77 Copyright &copy; 1997-2005 University of Cambridge.
236 nigel 75 <p>
237     Return to the <a href="index.html">PCRE index page</a>.
238     </p>

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