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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 ph10 111 <p>
11 nigel 75 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 ph10 111 <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 ph10 99 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 nigel 63 </ul>
23 nigel 75 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24 nigel 63 <P>
25     The PCRE library is a set of functions that implement regular expression
26     pattern matching using the same syntax and semantics as Perl, with just a few
27 ph10 345 differences. Certain features that appeared in Python and PCRE before they
28     appeared in Perl are also available using the Python syntax. There is also some
29     support for certain .NET and Oniguruma syntax items, and there is an option for
30     requesting some minor changes that give better JavaScript compatibility.
31 nigel 63 </P>
32     <P>
33 nigel 93 The current implementation of PCRE (release 7.x) corresponds approximately with
34     Perl 5.10, including support for UTF-8 encoded strings and Unicode general
35     category properties. However, UTF-8 and Unicode support has to be explicitly
36     enabled; it is not the default. The Unicode tables correspond to Unicode
37     release 5.0.0.
38     </P>
39     <P>
40     In addition to the Perl-compatible matching function, PCRE contains an
41 nigel 77 alternative matching function that matches the same compiled patterns in a
42     different way. In certain circumstances, the alternative function has some
43     advantages. For a discussion of the two matching algorithms, see the
44     <a href="pcrematching.html"><b>pcrematching</b></a>
45     page.
46     </P>
47     <P>
48 nigel 75 PCRE is written in C and released as a C library. A number of people have
49 nigel 77 written wrappers and interfaces of various kinds. In particular, Google Inc.
50     have provided a comprehensive C++ wrapper. This is now included as part of the
51     PCRE distribution. The
52     <a href="pcrecpp.html"><b>pcrecpp</b></a>
53     page has details of this interface. Other people's contributions can be found
54     in the <i>Contrib</i> directory at the primary FTP site, which is:
55 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>
56 nigel 63 </P>
57     <P>
58     Details of exactly which Perl regular expression features are and are not
59     supported by PCRE are given in separate documents. See the
60     <a href="pcrepattern.html"><b>pcrepattern</b></a>
61     and
62     <a href="pcrecompat.html"><b>pcrecompat</b></a>
63 ph10 208 pages. There is a syntax summary in the
64     <a href="pcresyntax.html"><b>pcresyntax</b></a>
65     page.
66 nigel 63 </P>
67     <P>
68     Some features of PCRE can be included, excluded, or changed when the library is
69     built. The
70     <a href="pcre_config.html"><b>pcre_config()</b></a>
71     function makes it possible for a client to discover which features are
72 nigel 75 available. The features themselves are described in the
73     <a href="pcrebuild.html"><b>pcrebuild</b></a>
74     page. Documentation about building PCRE for various operating systems can be
75     found in the <b>README</b> file in the source distribution.
76 nigel 63 </P>
77 nigel 77 <P>
78     The library contains a number of undocumented internal functions and data
79     tables that are used by more than one of the exported external functions, but
80     which are not intended for use by external callers. Their names all begin with
81 nigel 83 "_pcre_", which hopefully will not provoke any name clashes. In some
82     environments, it is possible to control which external symbols are exported
83     when a shared library is built, and in these cases the undocumented symbols are
84     not exported.
85 nigel 77 </P>
86 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
87     <P>
88 nigel 75 The user documentation for PCRE comprises a number of different sections. In
89     the "man" format, each of these is a separate "man page". In the HTML format,
90     each is a separate page, linked from the index page. In the plain text format,
91     all the sections are concatenated, for ease of searching. The sections are as
92     follows:
93 nigel 63 <pre>
94     pcre this document
95 ph10 153 pcre-config show PCRE installation configuration information
96 nigel 77 pcreapi details of PCRE's native C API
97 nigel 63 pcrebuild options for building PCRE
98     pcrecallout details of the callout feature
99     pcrecompat discussion of Perl compatibility
100 nigel 77 pcrecpp details of the C++ wrapper
101 nigel 63 pcregrep description of the <b>pcregrep</b> command
102 nigel 77 pcrematching discussion of the two matching algorithms
103 nigel 75 pcrepartial details of the partial matching facility
104     pcrepattern syntax and semantics of supported regular expressions
105 ph10 208 pcresyntax quick syntax reference
106 nigel 63 pcreperform discussion of performance issues
107 nigel 77 pcreposix the POSIX-compatible C API
108 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
109 nigel 63 pcresample discussion of the sample program
110 nigel 91 pcrestack discussion of stack usage
111 nigel 75 pcretest description of the <b>pcretest</b> testing command
112     </pre>
113 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
114 nigel 77 C library function, listing its arguments and results.
115 nigel 63 </P>
116     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
117     <P>
118     There are some size limitations in PCRE but it is hoped that they will never in
119     practice be relevant.
120     </P>
121     <P>
122     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
123     compiled with the default internal linkage size of 2. If you want to process
124     regular expressions that are truly enormous, you can compile PCRE with an
125     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
126     distribution and the
127     <a href="pcrebuild.html"><b>pcrebuild</b></a>
128 nigel 75 documentation for details). In these cases the limit is substantially larger.
129 nigel 93 However, the speed of execution is slower.
130 nigel 63 </P>
131     <P>
132 ph10 208 All values in repeating quantifiers must be less than 65536.
133 nigel 63 </P>
134     <P>
135 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
136     no more than 65535 capturing subpatterns.
137 nigel 63 </P>
138     <P>
139 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
140     maximum number of named subpatterns is 10000.
141 nigel 91 </P>
142     <P>
143 nigel 63 The maximum length of a subject string is the largest positive number that an
144 nigel 77 integer variable can hold. However, when using the traditional matching
145     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
146     This means that the available stack space may limit the size of a subject
147 nigel 91 string that can be processed by certain patterns. For a discussion of stack
148     issues, see the
149     <a href="pcrestack.html"><b>pcrestack</b></a>
150     documentation.
151 nigel 75 <a name="utf8support"></a></P>
152     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
153 nigel 63 <P>
154 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
155     the UTF-8 format. For release 4.0 this was greatly extended to cover most
156     common requirements, and in release 5.0 additional support for Unicode general
157     category properties was added.
158 nigel 63 </P>
159     <P>
160     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
161     the code, and, in addition, you must call
162     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
163     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
164     subject strings that are matched against it are treated as UTF-8 strings
165     instead of just strings of bytes.
166     </P>
167     <P>
168     If you compile PCRE with UTF-8 support, but do not use it at run time, the
169     library will be a bit bigger, but the additional run time overhead is limited
170 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
171 nigel 63 </P>
172     <P>
173 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
174     support), the escape sequences \p{..}, \P{..}, and \X are supported.
175     The available properties that can be tested are limited to the general
176     category properties such as Lu for an upper case letter or Nd for a decimal
177 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
178     properties Any and L&. A full list is given in the
179 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
180 nigel 87 documentation. Only the short names for properties are supported. For example,
181     \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
182     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
183     compatibility with Perl 5.6. PCRE does not support this.
184 ph10 211 <a name="utf8strings"></a></P>
185     <br><b>
186     Validity of UTF-8 strings
187     </b><br>
188     <P>
189     When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
190     are (by default) checked for validity on entry to the relevant functions. From
191     release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
192     themselves derived from the Unicode specification. Earlier releases of PCRE
193     followed the rules of RFC 2279, which allows the full range of 31-bit values (0
194     to 0x7FFFFFFF). The current check allows only values in the range U+0 to
195     U+10FFFF, excluding U+D800 to U+DFFF.
196 nigel 75 </P>
197     <P>
198 ph10 211 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
199     Unicode Standard says this: "The Low Surrogate Area does not contain any
200     character assignments, consequently no character code charts or namelists are
201     provided for this area. Surrogates are reserved for use with UTF-16 and then
202     must be used in pairs." The code points that are encoded by UTF-16 pairs are
203     available as independent code points in the UTF-8 encoding. (In other words,
204     the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
205     UTF-8.)
206 nigel 63 </P>
207     <P>
208 ph10 211 If an invalid UTF-8 string is passed to PCRE, an error return
209     (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
210     your strings are valid, and therefore want to skip these checks in order to
211     improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
212     at run time, PCRE assumes that the pattern or subject it is given
213     (respectively) contains only valid UTF-8 codes. In this case, it does not
214     diagnose an invalid UTF-8 string.
215 nigel 63 </P>
216     <P>
217 ph10 211 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
218     happens depends on why the string is invalid. If the string conforms to the
219     "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
220     in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
221     test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
222     rules of RFC 2279. However, if the string does not even conform to RFC 2279,
223     the result is undefined. Your program may crash.
224     </P>
225     <P>
226     If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
227     encoded in a UTF-8-like manner as per the old RFC, you can set
228     PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
229     situation, you will have to apply your own validity check.
230     </P>
231     <br><b>
232     General comments about UTF-8 mode
233     </b><br>
234     <P>
235     1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
236 nigel 87 UTF-8 character if the value is greater than 127.
237 nigel 63 </P>
238     <P>
239 ph10 211 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
240 nigel 91 characters for values greater than \177.
241     </P>
242     <P>
243 ph10 211 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
244 nigel 63 bytes, for example: \x{100}{3}.
245     </P>
246     <P>
247 ph10 211 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
248 nigel 63 </P>
249     <P>
250 ph10 211 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
251 nigel 77 but its use can lead to some strange effects. This facility is not available in
252     the alternative matching function, <b>pcre_dfa_exec()</b>.
253 nigel 63 </P>
254     <P>
255 ph10 211 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
256 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
257     digits, spaces, or word characters remain the same set as before, all with
258 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
259     property support, because to do otherwise would slow down PCRE in many common
260     cases. If you really want to test for a wider sense of, say, "digit", you
261     must use Unicode property tests such as \p{Nd}.
262 nigel 63 </P>
263     <P>
264 ph10 211 7. Similarly, characters that match the POSIX named character classes are all
265 nigel 75 low-valued characters.
266 nigel 63 </P>
267     <P>
268 ph10 211 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
269 ph10 182 (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
270     </P>
271     <P>
272 ph10 211 9. Case-insensitive matching applies only to characters whose values are less
273 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
274     property support is available, PCRE still uses its own character tables when
275     checking the case of low-valued characters, so as not to degrade performance.
276     The Unicode property information is used only for characters with higher
277 nigel 87 values. Even when Unicode property support is available, PCRE supports
278     case-insensitive matching only when there is a one-to-one mapping between a
279     letter's cases. There are a small number of many-to-one mappings in Unicode;
280     these are not supported by PCRE.
281 nigel 63 </P>
282     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
283     <P>
284 nigel 77 Philip Hazel
285 nigel 63 <br>
286 ph10 99 University Computing Service
287 nigel 63 <br>
288 nigel 93 Cambridge CB2 3QH, England.
289 ph10 99 <br>
290 nigel 77 </P>
291     <P>
292     Putting an actual email address here seems to have been a spam magnet, so I've
293 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
294     two digits 10, at the domain cam.ac.uk.
295 ph10 99 </P>
296     <br><a name="SEC6" href="#TOC1">REVISION</a><br>
297     <P>
298 ph10 345 Last updated: 12 April 2008
299 nigel 63 <br>
300 ph10 345 Copyright &copy; 1997-2008 University of Cambridge.
301 ph10 99 <br>
302 nigel 75 <p>
303     Return to the <a href="index.html">PCRE index page</a>.
304     </p>

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