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revision 450 by ph10, Wed Sep 16 10:56:40 2009 UTC revision 457 by ph10, Sat Oct 3 16:24:08 2009 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 mainly with respect to  regular expressions. The differences described here are with respect to Perl
9  Perl 5.8, though PCRE versions 7.0 and later contain some features that are  5.10.
 in Perl 5.10.  
10  .P  .P
11  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
12  it does have are given in the  it does have are given in the
# Line 69  The \eQ...\eE sequence is recognized bot Line 68  The \eQ...\eE sequence is recognized bot
68  .P  .P
69  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
71  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
73  the  the
74  .\" HREF  .\" HREF
# Line 78  the Line 77  the
77  documentation for details.  documentation for details.
78  .P  .P
79  9. Subpatterns that are called recursively or as "subroutines" are always  9. Subpatterns that are called recursively or as "subroutines" are always
80  treated as atomic groups in PCRE. This is like Python, but unlike Perl.  treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81    is a discussion of an example that explains this in more detail in the
82    .\" HTML <a href="pcrepattern.html#recursiondifference">
83    .\" </a>
84    section on recursion differences from Perl
85    .\"
86    in the
87    .\" HREF
88    \fBpcrepattern\fP
89    .\"
90    page.
91  .P  .P
92  10. There are some differences that are concerned with the settings of captured  10. There are some differences that are concerned with the settings of captured
93  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
# Line 88  the pattern /^(a(b)?)+$/ in Perl leaves Line 97  the pattern /^(a(b)?)+$/ in Perl leaves
97  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
98  argument. PCRE does not support (*MARK).  argument. PCRE does not support (*MARK).
99  .P  .P
100  12. PCRE provides some extensions to the Perl regular expression facilities.  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101  Perl 5.10 will include new features that are not in earlier versions, some of  names is not as general as Perl's. This is a consequence of the fact the PCRE
102  which (such as named parentheses) have been in PCRE for some time. This list is  works internally just with numbers, using an external table to translate
103  with respect to Perl 5.10:  between numbers and names. The following are some specific differences:
104  .sp  .sp
105  (a) Although lookbehind assertions must match fixed length strings, each  (a) A pattern such as (?|(?<a>A)|(?<b)B), where the two capturing
106  alternative branch of a lookbehind assertion can match a different length of  parentheses have the same number but different names, is not supported, and
107  string. Perl requires them all to have the same length.  causes an error at compile time. If it were allowed, it would not be possible
108    to distinguish which parentheses matched, because both names map to capturing
109    subpattern number 1. To avoid this confusing situation, an error is given at
110    compile time.
111    .sp
112    (b) A condition test for a subpattern with a name that is duplicated gives
113    unpredictable results. For example, when the pattern
114    (?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is
115    required), the condition test (?('a') is set to test whether subpattern 1 has
116    matched, ignoring subpattern 2, even though it has the same name.
117    .P
118    13. PCRE provides some extensions to the Perl regular expression facilities.
119    Perl 5.10 includes new features that are not in earlier versions of Perl, some
120    of which (such as named parentheses) have been in PCRE for some time. This list
121    is with respect to Perl 5.10:
122    .sp
123    (a) Although lookbehind assertions in PCRE must match fixed length strings,
124    each alternative branch of a lookbehind assertion can match a different length
125    of string. Perl requires them all to have the same length.
126  .sp  .sp
127  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
128  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
# Line 145  Cambridge CB2 3QH, England. Line 172  Cambridge CB2 3QH, England.
172  .rs  .rs
173  .sp  .sp
174  .nf  .nf
175  Last updated: 16 September 2009  Last updated: 03 October 2009
176  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
177  .fi  .fi
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