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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 579 by ph10, Wed Nov 24 17:39:25 2010 UTC
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
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 with respect to Perl
9  5.10.  versions 5.10 and above.
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 77  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. There  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  is a discussion of an example that explains this in more detail in the
82  .\" HTML <a href="pcrepattern.html#recursiondifference">  .\" HTML <a href="pcrepattern.html#recursiondifference">
83  .\" </a>  .\" </a>
# Line 93  page. Line 93  page.
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
94  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".
95  .P  .P
96  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),  11. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
97  (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an  names is not as general as Perl's. This is a consequence of the fact the PCRE
98  argument. PCRE does not support (*MARK).  works internally just with numbers, using an external table to translate
99  .P  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
100  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern  where the two capturing parentheses have the same number but different names,
101  names is not as general as Perl's. This is a consequence of the fact the PCRE  is not supported, and causes an error at compile time. If it were allowed, it
102  works internally just with numbers, using an external table to translate  would not be possible to distinguish which parentheses matched, because both
103  between numbers and names. The following are some specific differences:  names map to capturing subpattern number 1. To avoid this confusing situation,
104  .sp  an error is given at compile time.
105  (a) After matching a pattern such as (?|(?<a>A)|(?<b)B) where the two capturing  .P
106  parentheses have the same number but different names, it is not possible to  12. Perl recognizes comments in some places that PCRE doesn't, for example,
107  distinguish which parentheses matched, because both names map to capturing  between the ( and ? at the start of a subpattern.
 subpattern number 1.  
 .sp  
 (b) A condition test for a subpattern with a name that is duplicated gives  
 unpredictable results. For example, when the pattern  
 (?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is  
 required), the condition test (?('a') is set to test whether subpattern 1 has  
 matched, ignoring subpattern 2, even though it has the same name.  
108  .P  .P
109  13. PCRE provides some extensions to the Perl regular expression facilities.  13. PCRE provides some extensions to the Perl regular expression facilities.
110  Perl 5.10 includes new features that are not in earlier versions of Perl, some  Perl 5.10 includes new features that are not in earlier versions of Perl, some
# Line 170  Cambridge CB2 3QH, England. Line 163  Cambridge CB2 3QH, England.
163  .rs  .rs
164  .sp  .sp
165  .nf  .nf
166  Last updated: 29 September 2009  Last updated: 31 October 2010
167  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
168  .fi  .fi

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