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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 91 pcrestack discussion of stack usage
99 nigel 75 pcretest description of the <b>pcretest</b> testing command
100     </pre>
101 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
102 nigel 77 C library function, listing its arguments and results.
103 nigel 63 </P>
104     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
105     <P>
106     There are some size limitations in PCRE but it is hoped that they will never in
107     practice be relevant.
108     </P>
109     <P>
110     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
111     compiled with the default internal linkage size of 2. If you want to process
112     regular expressions that are truly enormous, you can compile PCRE with an
113     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
114     distribution and the
115     <a href="pcrebuild.html"><b>pcrebuild</b></a>
116 nigel 75 documentation for details). In these cases the limit is substantially larger.
117 nigel 63 However, the speed of execution will be slower.
118     </P>
119     <P>
120 nigel 91 All values in repeating quantifiers must be less than 65536. The maximum
121     compiled length of subpattern with an explicit repeat count is 30000 bytes. The
122     maximum number of capturing subpatterns is 65535.
123 nigel 63 </P>
124     <P>
125     There is no limit to the number of non-capturing subpatterns, but the maximum
126     depth of nesting of all kinds of parenthesized subpattern, including capturing
127     subpatterns, assertions, and other types of subpattern, is 200.
128     </P>
129     <P>
130 nigel 91 The maximum length of name for a named subpattern is 32, and the maximum number
131     of named subpatterns is 10000.
132     </P>
133     <P>
134 nigel 63 The maximum length of a subject string is the largest positive number that an
135 nigel 77 integer variable can hold. However, when using the traditional matching
136     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
137     This means that the available stack space may limit the size of a subject
138 nigel 91 string that can be processed by certain patterns. For a discussion of stack
139     issues, see the
140     <a href="pcrestack.html"><b>pcrestack</b></a>
141     documentation.
142 nigel 75 <a name="utf8support"></a></P>
143     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
144 nigel 63 <P>
145 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
146     the UTF-8 format. For release 4.0 this was greatly extended to cover most
147     common requirements, and in release 5.0 additional support for Unicode general
148     category properties was added.
149 nigel 63 </P>
150     <P>
151     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
152     the code, and, in addition, you must call
153     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
154     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
155     subject strings that are matched against it are treated as UTF-8 strings
156     instead of just strings of bytes.
157     </P>
158     <P>
159     If you compile PCRE with UTF-8 support, but do not use it at run time, the
160     library will be a bit bigger, but the additional run time overhead is limited
161     to testing the PCRE_UTF8 flag in several places, so should not be very large.
162     </P>
163     <P>
164 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
165     support), the escape sequences \p{..}, \P{..}, and \X are supported.
166     The available properties that can be tested are limited to the general
167     category properties such as Lu for an upper case letter or Nd for a decimal
168 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
169     properties Any and L&. A full list is given in the
170 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
171 nigel 87 documentation. Only the short names for properties are supported. For example,
172     \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
173     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
174     compatibility with Perl 5.6. PCRE does not support this.
175 nigel 75 </P>
176     <P>
177 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
178     </P>
179     <P>
180 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
181     are checked for validity on entry to the relevant functions. If an invalid
182     UTF-8 string is passed, an error return is given. In some situations, you may
183     already know that your strings are valid, and therefore want to skip these
184     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
185     at compile time or at run time, PCRE assumes that the pattern or subject it
186     is given (respectively) contains only valid UTF-8 codes. In this case, it does
187     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
188     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
189     may crash.
190 nigel 63 </P>
191     <P>
192 nigel 87 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
193     UTF-8 character if the value is greater than 127.
194 nigel 63 </P>
195     <P>
196 nigel 91 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
197     characters for values greater than \177.
198     </P>
199     <P>
200     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
201 nigel 63 bytes, for example: \x{100}{3}.
202     </P>
203     <P>
204 nigel 91 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
205 nigel 63 </P>
206     <P>
207 nigel 91 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
208 nigel 77 but its use can lead to some strange effects. This facility is not available in
209     the alternative matching function, <b>pcre_dfa_exec()</b>.
210 nigel 63 </P>
211     <P>
212 nigel 91 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
213 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
214     digits, spaces, or word characters remain the same set as before, all with
215 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
216     property support, because to do otherwise would slow down PCRE in many common
217     cases. If you really want to test for a wider sense of, say, "digit", you
218     must use Unicode property tests such as \p{Nd}.
219 nigel 63 </P>
220     <P>
221 nigel 91 8. Similarly, characters that match the POSIX named character classes are all
222 nigel 75 low-valued characters.
223 nigel 63 </P>
224     <P>
225 nigel 91 9. Case-insensitive matching applies only to characters whose values are less
226 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
227     property support is available, PCRE still uses its own character tables when
228     checking the case of low-valued characters, so as not to degrade performance.
229     The Unicode property information is used only for characters with higher
230 nigel 87 values. Even when Unicode property support is available, PCRE supports
231     case-insensitive matching only when there is a one-to-one mapping between a
232     letter's cases. There are a small number of many-to-one mappings in Unicode;
233     these are not supported by PCRE.
234 nigel 63 </P>
235     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
236     <P>
237 nigel 77 Philip Hazel
238 nigel 63 <br>
239     University Computing Service,
240     <br>
241     Cambridge CB2 3QG, England.
242 nigel 77 </P>
243     <P>
244     Putting an actual email address here seems to have been a spam magnet, so I've
245     taken it away. If you want to email me, use my initial and surname, separated
246     by a dot, at the domain ucs.cam.ac.uk.
247 nigel 91 Last updated: 05 June 2006
248 nigel 63 <br>
249 nigel 87 Copyright &copy; 1997-2006 University of Cambridge.
250 nigel 75 <p>
251     Return to the <a href="index.html">PCRE index page</a>.
252     </p>

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