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Update UTF-8 validity check and documentation.
1 <html>
2 <head>
3 <title>pcre specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <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 <ul>
16 <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17 <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18 <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19 <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20 <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21 <li><a name="TOC6" href="#SEC6">REVISION</a>
22 </ul>
23 <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24 <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 differences. (Certain features that appeared in Python and PCRE before they
28 appeared in Perl are also available using the Python syntax.)
29 </P>
30 <P>
31 The current implementation of PCRE (release 7.x) corresponds approximately with
32 Perl 5.10, including support for UTF-8 encoded strings and Unicode general
33 category properties. However, UTF-8 and Unicode support has to be explicitly
34 enabled; it is not the default. The Unicode tables correspond to Unicode
35 release 5.0.0.
36 </P>
37 <P>
38 In addition to the Perl-compatible matching function, PCRE contains an
39 alternative matching function that matches the same compiled patterns in a
40 different way. In certain circumstances, the alternative function has some
41 advantages. For a discussion of the two matching algorithms, see the
42 <a href="pcrematching.html"><b>pcrematching</b></a>
43 page.
44 </P>
45 <P>
46 PCRE is written in C and released as a C library. A number of people have
47 written wrappers and interfaces of various kinds. In particular, Google Inc.
48 have provided a comprehensive C++ wrapper. This is now included as part of the
49 PCRE distribution. The
50 <a href="pcrecpp.html"><b>pcrecpp</b></a>
51 page has details of this interface. Other people's contributions can be found
52 in the <i>Contrib</i> directory at the primary FTP site, which is:
53 <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
54 </P>
55 <P>
56 Details of exactly which Perl regular expression features are and are not
57 supported by PCRE are given in separate documents. See the
58 <a href="pcrepattern.html"><b>pcrepattern</b></a>
59 and
60 <a href="pcrecompat.html"><b>pcrecompat</b></a>
61 pages. There is a syntax summary in the
62 <a href="pcresyntax.html"><b>pcresyntax</b></a>
63 page.
64 </P>
65 <P>
66 Some features of PCRE can be included, excluded, or changed when the library is
67 built. The
68 <a href="pcre_config.html"><b>pcre_config()</b></a>
69 function makes it possible for a client to discover which features are
70 available. The features themselves are described in the
71 <a href="pcrebuild.html"><b>pcrebuild</b></a>
72 page. Documentation about building PCRE for various operating systems can be
73 found in the <b>README</b> file in the source distribution.
74 </P>
75 <P>
76 The library contains a number of undocumented internal functions and data
77 tables that are used by more than one of the exported external functions, but
78 which are not intended for use by external callers. Their names all begin with
79 "_pcre_", which hopefully will not provoke any name clashes. In some
80 environments, it is possible to control which external symbols are exported
81 when a shared library is built, and in these cases the undocumented symbols are
82 not exported.
83 </P>
84 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
85 <P>
86 The user documentation for PCRE comprises a number of different sections. In
87 the "man" format, each of these is a separate "man page". In the HTML format,
88 each is a separate page, linked from the index page. In the plain text format,
89 all the sections are concatenated, for ease of searching. The sections are as
90 follows:
91 <pre>
92 pcre this document
93 pcre-config show PCRE installation configuration information
94 pcreapi details of PCRE's native C API
95 pcrebuild options for building PCRE
96 pcrecallout details of the callout feature
97 pcrecompat discussion of Perl compatibility
98 pcrecpp details of the C++ wrapper
99 pcregrep description of the <b>pcregrep</b> command
100 pcrematching discussion of the two matching algorithms
101 pcrepartial details of the partial matching facility
102 pcrepattern syntax and semantics of supported regular expressions
103 pcresyntax quick syntax reference
104 pcreperform discussion of performance issues
105 pcreposix the POSIX-compatible C API
106 pcreprecompile details of saving and re-using precompiled patterns
107 pcresample discussion of the sample program
108 pcrestack discussion of stack usage
109 pcretest description of the <b>pcretest</b> testing command
110 </pre>
111 In addition, in the "man" and HTML formats, there is a short page for each
112 C library function, listing its arguments and results.
113 </P>
114 <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
115 <P>
116 There are some size limitations in PCRE but it is hoped that they will never in
117 practice be relevant.
118 </P>
119 <P>
120 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
121 compiled with the default internal linkage size of 2. If you want to process
122 regular expressions that are truly enormous, you can compile PCRE with an
123 internal linkage size of 3 or 4 (see the <b>README</b> file in the source
124 distribution and the
125 <a href="pcrebuild.html"><b>pcrebuild</b></a>
126 documentation for details). In these cases the limit is substantially larger.
127 However, the speed of execution is slower.
128 </P>
129 <P>
130 All values in repeating quantifiers must be less than 65536.
131 </P>
132 <P>
133 There is no limit to the number of parenthesized subpatterns, but there can be
134 no more than 65535 capturing subpatterns.
135 </P>
136 <P>
137 The maximum length of name for a named subpattern is 32 characters, and the
138 maximum number of named subpatterns is 10000.
139 </P>
140 <P>
141 The maximum length of a subject string is the largest positive number that an
142 integer variable can hold. However, when using the traditional matching
143 function, PCRE uses recursion to handle subpatterns and indefinite repetition.
144 This means that the available stack space may limit the size of a subject
145 string that can be processed by certain patterns. For a discussion of stack
146 issues, see the
147 <a href="pcrestack.html"><b>pcrestack</b></a>
148 documentation.
149 <a name="utf8support"></a></P>
150 <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
151 <P>
152 From release 3.3, PCRE has had some support for character strings encoded in
153 the UTF-8 format. For release 4.0 this was greatly extended to cover most
154 common requirements, and in release 5.0 additional support for Unicode general
155 category properties was added.
156 </P>
157 <P>
158 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
159 the code, and, in addition, you must call
160 <a href="pcre_compile.html"><b>pcre_compile()</b></a>
161 with the PCRE_UTF8 option flag. When you do this, both the pattern and any
162 subject strings that are matched against it are treated as UTF-8 strings
163 instead of just strings of bytes.
164 </P>
165 <P>
166 If you compile PCRE with UTF-8 support, but do not use it at run time, the
167 library will be a bit bigger, but the additional run time overhead is limited
168 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
169 </P>
170 <P>
171 If PCRE is built with Unicode character property support (which implies UTF-8
172 support), the escape sequences \p{..}, \P{..}, and \X are supported.
173 The available properties that can be tested are limited to the general
174 category properties such as Lu for an upper case letter or Nd for a decimal
175 number, the Unicode script names such as Arabic or Han, and the derived
176 properties Any and L&. A full list is given in the
177 <a href="pcrepattern.html"><b>pcrepattern</b></a>
178 documentation. Only the short names for properties are supported. For example,
179 \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
180 Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
181 compatibility with Perl 5.6. PCRE does not support this.
182 <a name="utf8strings"></a></P>
183 <br><b>
184 Validity of UTF-8 strings
185 </b><br>
186 <P>
187 When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
188 are (by default) checked for validity on entry to the relevant functions. From
189 release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
190 themselves derived from the Unicode specification. Earlier releases of PCRE
191 followed the rules of RFC 2279, which allows the full range of 31-bit values (0
192 to 0x7FFFFFFF). The current check allows only values in the range U+0 to
193 U+10FFFF, excluding U+D800 to U+DFFF.
194 </P>
195 <P>
196 The excluded code points are the "Low Surrogate Area" of Unicode, of which the
197 Unicode Standard says this: "The Low Surrogate Area does not contain any
198 character assignments, consequently no character code charts or namelists are
199 provided for this area. Surrogates are reserved for use with UTF-16 and then
200 must be used in pairs." The code points that are encoded by UTF-16 pairs are
201 available as independent code points in the UTF-8 encoding. (In other words,
202 the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
203 UTF-8.)
204 </P>
205 <P>
206 If an invalid UTF-8 string is passed to PCRE, an error return
207 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
208 your strings are valid, and therefore want to skip these checks in order to
209 improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
210 at run time, PCRE assumes that the pattern or subject it is given
211 (respectively) contains only valid UTF-8 codes. In this case, it does not
212 diagnose an invalid UTF-8 string.
213 </P>
214 <P>
215 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
216 happens depends on why the string is invalid. If the string conforms to the
217 "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
218 in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
219 test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
220 rules of RFC 2279. However, if the string does not even conform to RFC 2279,
221 the result is undefined. Your program may crash.
222 </P>
223 <P>
224 If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
225 encoded in a UTF-8-like manner as per the old RFC, you can set
226 PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
227 situation, you will have to apply your own validity check.
228 </P>
229 <br><b>
230 General comments about UTF-8 mode
231 </b><br>
232 <P>
233 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
234 UTF-8 character if the value is greater than 127.
235 </P>
236 <P>
237 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
238 characters for values greater than \177.
239 </P>
240 <P>
241 3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
242 bytes, for example: \x{100}{3}.
243 </P>
244 <P>
245 4. The dot metacharacter matches one UTF-8 character instead of a single byte.
246 </P>
247 <P>
248 5. The escape sequence \C can be used to match a single byte in UTF-8 mode,
249 but its use can lead to some strange effects. This facility is not available in
250 the alternative matching function, <b>pcre_dfa_exec()</b>.
251 </P>
252 <P>
253 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
254 test characters of any code value, but the characters that PCRE recognizes as
255 digits, spaces, or word characters remain the same set as before, all with
256 values less than 256. This remains true even when PCRE includes Unicode
257 property support, because to do otherwise would slow down PCRE in many common
258 cases. If you really want to test for a wider sense of, say, "digit", you
259 must use Unicode property tests such as \p{Nd}.
260 </P>
261 <P>
262 7. Similarly, characters that match the POSIX named character classes are all
263 low-valued characters.
264 </P>
265 <P>
266 8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
267 (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
268 </P>
269 <P>
270 9. Case-insensitive matching applies only to characters whose values are less
271 than 128, unless PCRE is built with Unicode property support. Even when Unicode
272 property support is available, PCRE still uses its own character tables when
273 checking the case of low-valued characters, so as not to degrade performance.
274 The Unicode property information is used only for characters with higher
275 values. Even when Unicode property support is available, PCRE supports
276 case-insensitive matching only when there is a one-to-one mapping between a
277 letter's cases. There are a small number of many-to-one mappings in Unicode;
278 these are not supported by PCRE.
279 </P>
280 <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
281 <P>
282 Philip Hazel
283 <br>
284 University Computing Service
285 <br>
286 Cambridge CB2 3QH, England.
287 <br>
288 </P>
289 <P>
290 Putting an actual email address here seems to have been a spam magnet, so I've
291 taken it away. If you want to email me, use my two initials, followed by the
292 two digits 10, at the domain cam.ac.uk.
293 </P>
294 <br><a name="SEC6" href="#TOC1">REVISION</a><br>
295 <P>
296 Last updated: 09 August 2007
297 <br>
298 Copyright &copy; 1997-2007 University of Cambridge.
299 <br>
300 <p>
301 Return to the <a href="index.html">PCRE index page</a>.
302 </p>

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