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revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 345 by ph10, Mon Apr 28 15:10:02 2008 UTC
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
6  .sp  .sp
7  The PCRE library is a set of functions that implement regular expression  The PCRE library is a set of functions that implement regular expression
8  pattern matching using the same syntax and semantics as Perl, with just a few  pattern matching using the same syntax and semantics as Perl, with just a few
9  differences. The current implementation of PCRE (release 6.x) corresponds  differences. Certain features that appeared in Python and PCRE before they
10  approximately with Perl 5.8, including support for UTF-8 encoded strings and  appeared in Perl are also available using the Python syntax. There is also some
11  Unicode general category properties. However, this support has to be explicitly  support for certain .NET and Oniguruma syntax items, and there is an option for
12  enabled; it is not the default.  requesting some minor changes that give better JavaScript compatibility.
13    .P
14    The current implementation of PCRE (release 7.x) corresponds approximately with
15    Perl 5.10, including support for UTF-8 encoded strings and Unicode general
16    category properties. However, UTF-8 and Unicode support has to be explicitly
17    enabled; it is not the default. The Unicode tables correspond to Unicode
18    release 5.0.0.
19  .P  .P
20  In addition to the Perl-compatible matching function, PCRE also contains an  In addition to the Perl-compatible matching function, PCRE contains an
21  alternative matching function that matches the same compiled patterns in a  alternative matching function that matches the same compiled patterns in a
22  different way. In certain circumstances, the alternative function has some  different way. In certain circumstances, the alternative function has some
23  advantages. For a discussion of the two matching algorithms, see the  advantages. For a discussion of the two matching algorithms, see the
# Line 43  and Line 49  and
49  .\" HREF  .\" HREF
50  \fBpcrecompat\fR  \fBpcrecompat\fR
51  .\"  .\"
52  pages.  pages. There is a syntax summary in the
53    .\" HREF
54    \fBpcresyntax\fR
55    .\"
56    page.
57  .P  .P
58  Some features of PCRE can be included, excluded, or changed when the library is  Some features of PCRE can be included, excluded, or changed when the library is
59  built. The  built. The
# Line 77  all the sections are concatenated, for e Line 87  all the sections are concatenated, for e
87  follows:  follows:
88  .sp  .sp
89    pcre              this document    pcre              this document
90      pcre-config       show PCRE installation configuration information
91    pcreapi           details of PCRE's native C API    pcreapi           details of PCRE's native C API
92    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
93    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
# Line 88  follows: Line 99  follows:
99  .\" JOIN  .\" JOIN
100    pcrepattern       syntax and semantics of supported    pcrepattern       syntax and semantics of supported
101                        regular expressions                        regular expressions
102      pcresyntax        quick syntax reference
103    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
104    pcreposix         the POSIX-compatible C API    pcreposix         the POSIX-compatible C API
105    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
106    pcresample        discussion of the sample program    pcresample        discussion of the sample program
107      pcrestack         discussion of stack usage
108    pcretest          description of the \fBpcretest\fP testing command    pcretest          description of the \fBpcretest\fP testing command
109  .sp  .sp
110  In addition, in the "man" and HTML formats, there is a short page for each  In addition, in the "man" and HTML formats, there is a short page for each
# Line 113  distribution and the Line 126  distribution and the
126  \fBpcrebuild\fP  \fBpcrebuild\fP
127  .\"  .\"
128  documentation for details). In these cases the limit is substantially larger.  documentation for details). In these cases the limit is substantially larger.
129  However, the speed of execution will be slower.  However, the speed of execution is slower.
130  .P  .P
131  All values in repeating quantifiers must be less than 65536.  All values in repeating quantifiers must be less than 65536.
 The maximum number of capturing subpatterns is 65535.  
132  .P  .P
133  There is no limit to the number of non-capturing subpatterns, but the maximum  There is no limit to the number of parenthesized subpatterns, but there can be
134  depth of nesting of all kinds of parenthesized subpattern, including capturing  no more than 65535 capturing subpatterns.
135  subpatterns, assertions, and other types of subpattern, is 200.  .P
136    The maximum length of name for a named subpattern is 32 characters, and the
137    maximum number of named subpatterns is 10000.
138  .P  .P
139  The maximum length of a subject string is the largest positive number that an  The maximum length of a subject string is the largest positive number that an
140  integer variable can hold. However, when using the traditional matching  integer variable can hold. However, when using the traditional matching
141  function, PCRE uses recursion to handle subpatterns and indefinite repetition.  function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142  This means that the available stack space may limit the size of a subject  This means that the available stack space may limit the size of a subject
143  string that can be processed by certain patterns.  string that can be processed by certain patterns. For a discussion of stack
144  .sp  issues, see the
145    .\" HREF
146    \fBpcrestack\fP
147    .\"
148    documentation.
149    .
150  .\" HTML <a name="utf8support"></a>  .\" HTML <a name="utf8support"></a>
151  .  .
152  .  .
# Line 150  instead of just strings of bytes. Line 169  instead of just strings of bytes.
169  .P  .P
170  If you compile PCRE with UTF-8 support, but do not use it at run time, the  If you compile PCRE with UTF-8 support, but do not use it at run time, the
171  library will be a bit bigger, but the additional run time overhead is limited  library will be a bit bigger, but the additional run time overhead is limited
172  to testing the PCRE_UTF8 flag in several places, so should not be very large.  to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173  .P  .P
174  If PCRE is built with Unicode character property support (which implies UTF-8  If PCRE is built with Unicode character property support (which implies UTF-8
175  support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.  support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
# Line 165  documentation. Only the short names for Line 184  documentation. Only the short names for
184  \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.  \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
185  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for  Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186  compatibility with Perl 5.6. PCRE does not support this.  compatibility with Perl 5.6. PCRE does not support this.
187  .P  .
188  The following comments apply when PCRE is running in UTF-8 mode:  .\" HTML <a name="utf8strings"></a>
189  .P  .
190  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects  .SS "Validity of UTF-8 strings"
191  are checked for validity on entry to the relevant functions. If an invalid  .rs
192  UTF-8 string is passed, an error return is given. In some situations, you may  .sp
193  already know that your strings are valid, and therefore want to skip these  When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
194  checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag  are (by default) checked for validity on entry to the relevant functions. From
195  at compile time or at run time, PCRE assumes that the pattern or subject it  release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
196  is given (respectively) contains only valid UTF-8 codes. In this case, it does  themselves derived from the Unicode specification. Earlier releases of PCRE
197  not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to  followed the rules of RFC 2279, which allows the full range of 31-bit values (0
198  PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  to 0x7FFFFFFF). The current check allows only values in the range U+0 to
199  may crash.  U+10FFFF, excluding U+D800 to U+DFFF.
200  .P  .P
201  2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte  The excluded code points are the "Low Surrogate Area" of Unicode, of which the
202    Unicode Standard says this: "The Low Surrogate Area does not contain any
203    character assignments, consequently no character code charts or namelists are
204    provided for this area. Surrogates are reserved for use with UTF-16 and then
205    must be used in pairs." The code points that are encoded by UTF-16 pairs are
206    available as independent code points in the UTF-8 encoding. (In other words,
207    the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
208    UTF-8.)
209    .P
210    If an invalid UTF-8 string is passed to PCRE, an error return
211    (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
212    your strings are valid, and therefore want to skip these checks in order to
213    improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
214    at run time, PCRE assumes that the pattern or subject it is given
215    (respectively) contains only valid UTF-8 codes. In this case, it does not
216    diagnose an invalid UTF-8 string.
217    .P
218    If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
219    happens depends on why the string is invalid. If the string conforms to the
220    "old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
221    in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
222    test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
223    rules of RFC 2279. However, if the string does not even conform to RFC 2279,
224    the result is undefined. Your program may crash.
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    .
231    .SS "General comments about UTF-8 mode"
232    .rs
233    .sp
234    1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
235  UTF-8 character if the value is greater than 127.  UTF-8 character if the value is greater than 127.
236  .P  .P
237    2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
238    characters for values greater than \e177.
239    .P
240  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual  3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
241  bytes, for example: \ex{100}{3}.  bytes, for example: \ex{100}{3}.
242  .P  .P
# Line 202  must use Unicode property tests such as Line 257  must use Unicode property tests such as
257  7. Similarly, characters that match the POSIX named character classes are all  7. Similarly, characters that match the POSIX named character classes are all
258  low-valued characters.  low-valued characters.
259  .P  .P
260  8. Case-insensitive matching applies only to characters whose values are less  8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
261    (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
262    .P
263    9. Case-insensitive matching applies only to characters whose values are less
264  than 128, unless PCRE is built with Unicode property support. Even when Unicode  than 128, unless PCRE is built with Unicode property support. Even when Unicode
265  property support is available, PCRE still uses its own character tables when  property support is available, PCRE still uses its own character tables when
266  checking the case of low-valued characters, so as not to degrade performance.  checking the case of low-valued characters, so as not to degrade performance.
# Line 212  case-insensitive matching only when ther Line 270  case-insensitive matching only when ther
270  letter's cases. There are a small number of many-to-one mappings in Unicode;  letter's cases. There are a small number of many-to-one mappings in Unicode;
271  these are not supported by PCRE.  these are not supported by PCRE.
272  .  .
273    .
274  .SH AUTHOR  .SH AUTHOR
275  .rs  .rs
276  .sp  .sp
277    .nf
278  Philip Hazel  Philip Hazel
279  .br  University Computing Service
280  University Computing Service,  Cambridge CB2 3QH, England.
281  .br  .fi
 Cambridge CB2 3QG, England.  
282  .P  .P
283  Putting an actual email address here seems to have been a spam magnet, so I've  Putting an actual email address here seems to have been a spam magnet, so I've
284  taken it away. If you want to email me, use my initial and surname, separated  taken it away. If you want to email me, use my two initials, followed by the
285  by a dot, at the domain ucs.cam.ac.uk.  two digits 10, at the domain cam.ac.uk.
286    .
287    .
288    .SH REVISION
289    .rs
290  .sp  .sp
291  .in 0  .nf
292  Last updated: 24 January 2006  Last updated: 12 April 2008
293  .br  Copyright (c) 1997-2008 University of Cambridge.
294  Copyright (c) 1997-2006 University of Cambridge.  .fi

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