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Revision 197 - (hide annotations) (download) (as text)
Tue Jul 31 10:50:18 2007 UTC (5 years, 10 months ago) by ph10
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Abolish the NULLWSLIMIT error at the expense of using more stack when an 
unlimited repeat could match an empty string. Also, doc tidies for a test 
release.

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 nigel 93 differences. (Certain features that appeared in Python and PCRE before they
28     appeared in Perl are also available using the Python syntax.)
29 nigel 63 </P>
30     <P>
31 nigel 93 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 nigel 77 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 nigel 75 PCRE is written in C and released as a C library. A number of people have
47 nigel 77 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 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>
54 nigel 63 </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.
62     </P>
63     <P>
64     Some features of PCRE can be included, excluded, or changed when the library is
65     built. The
66     <a href="pcre_config.html"><b>pcre_config()</b></a>
67     function makes it possible for a client to discover which features are
68 nigel 75 available. The features themselves are described in the
69     <a href="pcrebuild.html"><b>pcrebuild</b></a>
70     page. Documentation about building PCRE for various operating systems can be
71     found in the <b>README</b> file in the source distribution.
72 nigel 63 </P>
73 nigel 77 <P>
74     The library contains a number of undocumented internal functions and data
75     tables that are used by more than one of the exported external functions, but
76     which are not intended for use by external callers. Their names all begin with
77 nigel 83 "_pcre_", which hopefully will not provoke any name clashes. In some
78     environments, it is possible to control which external symbols are exported
79     when a shared library is built, and in these cases the undocumented symbols are
80     not exported.
81 nigel 77 </P>
82 nigel 63 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
83     <P>
84 nigel 75 The user documentation for PCRE comprises a number of different sections. In
85     the "man" format, each of these is a separate "man page". In the HTML format,
86     each is a separate page, linked from the index page. In the plain text format,
87     all the sections are concatenated, for ease of searching. The sections are as
88     follows:
89 nigel 63 <pre>
90     pcre this document
91 ph10 153 pcre-config show PCRE installation configuration information
92 nigel 77 pcreapi details of PCRE's native C API
93 nigel 63 pcrebuild options for building PCRE
94     pcrecallout details of the callout feature
95     pcrecompat discussion of Perl compatibility
96 nigel 77 pcrecpp details of the C++ wrapper
97 nigel 63 pcregrep description of the <b>pcregrep</b> command
98 nigel 77 pcrematching discussion of the two matching algorithms
99 nigel 75 pcrepartial details of the partial matching facility
100     pcrepattern syntax and semantics of supported regular expressions
101 nigel 63 pcreperform discussion of performance issues
102 nigel 77 pcreposix the POSIX-compatible C API
103 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
104 nigel 63 pcresample discussion of the sample program
105 nigel 91 pcrestack discussion of stack usage
106 nigel 75 pcretest description of the <b>pcretest</b> testing command
107     </pre>
108 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
109 nigel 77 C library function, listing its arguments and results.
110 nigel 63 </P>
111     <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
112     <P>
113     There are some size limitations in PCRE but it is hoped that they will never in
114     practice be relevant.
115     </P>
116     <P>
117     The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
118     compiled with the default internal linkage size of 2. If you want to process
119     regular expressions that are truly enormous, you can compile PCRE with an
120     internal linkage size of 3 or 4 (see the <b>README</b> file in the source
121     distribution and the
122     <a href="pcrebuild.html"><b>pcrebuild</b></a>
123 nigel 75 documentation for details). In these cases the limit is substantially larger.
124 nigel 93 However, the speed of execution is slower.
125 nigel 63 </P>
126     <P>
127 nigel 91 All values in repeating quantifiers must be less than 65536. The maximum
128     compiled length of subpattern with an explicit repeat count is 30000 bytes. The
129     maximum number of capturing subpatterns is 65535.
130 nigel 63 </P>
131     <P>
132 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
133     no more than 65535 capturing subpatterns.
134 nigel 63 </P>
135     <P>
136 ph10 197 If a non-capturing subpattern with an unlimited repetition quantifier can match
137     an empty string, there is a limit of 1000 on the number of times it can be
138     repeated while not matching an empty string - if it does match an empty
139     string, the loop is immediately broken.
140     </P>
141     <P>
142 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
143     maximum number of named subpatterns is 10000.
144 nigel 91 </P>
145     <P>
146 nigel 63 The maximum length of a subject string is the largest positive number that an
147 nigel 77 integer variable can hold. However, when using the traditional matching
148     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
149     This means that the available stack space may limit the size of a subject
150 nigel 91 string that can be processed by certain patterns. For a discussion of stack
151     issues, see the
152     <a href="pcrestack.html"><b>pcrestack</b></a>
153     documentation.
154 nigel 75 <a name="utf8support"></a></P>
155     <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
156 nigel 63 <P>
157 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
158     the UTF-8 format. For release 4.0 this was greatly extended to cover most
159     common requirements, and in release 5.0 additional support for Unicode general
160     category properties was added.
161 nigel 63 </P>
162     <P>
163     In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
164     the code, and, in addition, you must call
165     <a href="pcre_compile.html"><b>pcre_compile()</b></a>
166     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
167     subject strings that are matched against it are treated as UTF-8 strings
168     instead of just strings of bytes.
169     </P>
170     <P>
171     If you compile PCRE with UTF-8 support, but do not use it at run time, the
172     library will be a bit bigger, but the additional run time overhead is limited
173 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
174 nigel 63 </P>
175     <P>
176 nigel 75 If PCRE is built with Unicode character property support (which implies UTF-8
177     support), the escape sequences \p{..}, \P{..}, and \X are supported.
178     The available properties that can be tested are limited to the general
179     category properties such as Lu for an upper case letter or Nd for a decimal
180 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
181     properties Any and L&. A full list is given in the
182 nigel 75 <a href="pcrepattern.html"><b>pcrepattern</b></a>
183 nigel 87 documentation. Only the short names for properties are supported. For example,
184     \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
185     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186     compatibility with Perl 5.6. PCRE does not support this.
187 nigel 75 </P>
188     <P>
189 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
190     </P>
191     <P>
192 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
193     are checked for validity on entry to the relevant functions. If an invalid
194     UTF-8 string is passed, an error return is given. In some situations, you may
195     already know that your strings are valid, and therefore want to skip these
196     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
197     at compile time or at run time, PCRE assumes that the pattern or subject it
198     is given (respectively) contains only valid UTF-8 codes. In this case, it does
199     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
200     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
201     may crash.
202 nigel 63 </P>
203     <P>
204 nigel 87 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
205     UTF-8 character if the value is greater than 127.
206 nigel 63 </P>
207     <P>
208 nigel 91 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
209     characters for values greater than \177.
210     </P>
211     <P>
212     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
213 nigel 63 bytes, for example: \x{100}{3}.
214     </P>
215     <P>
216 nigel 91 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
217 nigel 63 </P>
218     <P>
219 nigel 91 6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
220 nigel 77 but its use can lead to some strange effects. This facility is not available in
221     the alternative matching function, <b>pcre_dfa_exec()</b>.
222 nigel 63 </P>
223     <P>
224 nigel 91 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
225 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
226     digits, spaces, or word characters remain the same set as before, all with
227 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
228     property support, because to do otherwise would slow down PCRE in many common
229     cases. If you really want to test for a wider sense of, say, "digit", you
230     must use Unicode property tests such as \p{Nd}.
231 nigel 63 </P>
232     <P>
233 nigel 91 8. Similarly, characters that match the POSIX named character classes are all
234 nigel 75 low-valued characters.
235 nigel 63 </P>
236     <P>
237 ph10 182 9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
238     (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
239     </P>
240     <P>
241     10. Case-insensitive matching applies only to characters whose values are less
242 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
243     property support is available, PCRE still uses its own character tables when
244     checking the case of low-valued characters, so as not to degrade performance.
245     The Unicode property information is used only for characters with higher
246 nigel 87 values. Even when Unicode property support is available, PCRE supports
247     case-insensitive matching only when there is a one-to-one mapping between a
248     letter's cases. There are a small number of many-to-one mappings in Unicode;
249     these are not supported by PCRE.
250 nigel 63 </P>
251     <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
252     <P>
253 nigel 77 Philip Hazel
254 nigel 63 <br>
255 ph10 99 University Computing Service
256 nigel 63 <br>
257 nigel 93 Cambridge CB2 3QH, England.
258 ph10 99 <br>
259 nigel 77 </P>
260     <P>
261     Putting an actual email address here seems to have been a spam magnet, so I've
262 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
263     two digits 10, at the domain cam.ac.uk.
264 ph10 99 </P>
265     <br><a name="SEC6" href="#TOC1">REVISION</a><br>
266     <P>
267 ph10 197 Last updated: 30 July 2007
268 nigel 63 <br>
269 ph10 99 Copyright &copy; 1997-2007 University of Cambridge.
270     <br>
271 nigel 75 <p>
272     Return to the <a href="index.html">PCRE index page</a>.
273     </p>

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