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revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 231 by ph10, Tue Sep 11 11:15:33 2007 UTC
# Line 5  PCRE - Perl-compatible regular expressio Line 5  PCRE - Perl-compatible regular expressio
5  .rs  .rs
6  .sp  .sp
7  This document describes the differences in the ways that PCRE and Perl handle  This document describes the differences in the ways that PCRE and Perl handle
8  regular expressions. The differences described here are with respect to Perl  regular expressions. The differences described here are mainly with respect to
9  5.8.  Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10    expected to be in the forthcoming Perl 5.10.
11  .P  .P
12  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
13  it does have are given in the  it does have are given in the
# Line 63  following examples: Line 64  following examples:
64  .sp  .sp
65  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
66  .P  .P
67  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
68  constructions. However, there is support for recursive patterns using the  constructions. However, there is support for recursive patterns. This is not
69  non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
70  allows an external function to be called during pattern matching. See the  feature allows an external function to be called during pattern matching. See
71    the
72  .\" HREF  .\" HREF
73  \fBpcrecallout\fP  \fBpcrecallout\fP
74  .\"  .\"
75  documentation for details.  documentation for details.
76  .P  .P
77  9. There are some differences that are concerned with the settings of captured  9. Subpatterns that are called recursively or as "subroutines" are always
78    treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79    .P
80    10. There are some differences that are concerned with the settings of captured
81  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
82  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
83  .P  .P
84  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
85    (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
86    argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
87    parentheses, PCRE does not set that capture group; this is different to Perl.
88    .P
89    12. PCRE provides some extensions to the Perl regular expression facilities.
90    Perl 5.10 will include new features that are not in earlier versions, some of
91    which (such as named parentheses) have been in PCRE for some time. This list is
92    with respect to Perl 5.10:
93  .sp  .sp
94  (a) Although lookbehind assertions must match fixed length strings, each  (a) Although lookbehind assertions must match fixed length strings, each
95  alternative branch of a lookbehind assertion can match a different length of  alternative branch of a lookbehind assertion can match a different length of
# Line 86  string. Perl requires them all to have t Line 99  string. Perl requires them all to have t
99  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
100  .sp  .sp
101  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
102  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
103    (Perl can be made to issue a warning.)
104  .sp  .sp
105  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
106  inverted, that is, by default they are not greedy, but if followed by a  inverted, that is, by default they are not greedy, but if followed by a
# Line 98  only at the first matching position in t Line 112  only at the first matching position in t
112  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
113  options for \fBpcre_exec()\fP have no Perl equivalents.  options for \fBpcre_exec()\fP have no Perl equivalents.
114  .sp  .sp
115  (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
116  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
 support.)  
117  .sp  .sp
118  (h) PCRE supports named capturing substrings, using the Python syntax.  (h) The callout facility is PCRE-specific.
119  .sp  .sp
120  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  (i) The partial matching facility is PCRE-specific.
 package.  
121  .sp  .sp
122  (j) The (R) condition, for testing recursion, is a PCRE extension.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
123    different hosts that have the other endianness.
124  .sp  .sp
125  (k) The callout facility is PCRE-specific.  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
126    different way and is not Perl-compatible.
127  .sp  .sp
128  (l) The partial matching facility is PCRE-specific.  (l) PCRE recognizes some special sequences such as (*CR) at the start of
129    a pattern that set overall options that cannot be changed within the pattern.
130    .
131    .
132    .SH AUTHOR
133    .rs
134  .sp  .sp
135  (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on  .nf
136  different hosts that have the other endianness.  Philip Hazel
137    University Computing Service
138    Cambridge CB2 3QH, England.
139    .fi
140    .
141    .
142    .SH REVISION
143    .rs
144  .sp  .sp
145  (n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  .nf
146  different way and is not Perl-compatible.  Last updated: 11 September 2007
147  .P  Copyright (c) 1997-2007 University of Cambridge.
148  .in 0  .fi
 Last updated: 24 January 2006  
 .br  
 Copyright (c) 1997-2006 University of Cambridge.  

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