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revision 457 by ph10, Sat Oct 3 16:24:08 2009 UTC revision 596 by ph10, Mon May 2 10:47:52 2011 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 20  in the main Line 20  in the main
20  .\"  .\"
21  page.  page.
22  .P  .P
23  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  2. PCRE does not allow repeat quantifiers on assertions. Perl permits them, but
24  them, but they do not mean what you might think. For example, (?!a){3} does  they do not mean what you might think. For example, (?!a){3} does not assert
25  not assert that the next three characters are not "a". It just asserts that the  that the next three characters are not "a". It just asserts that the next
26  next character is not "a" three times.  character is not "a" three times.
27  .P  .P
28  3. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
29  counted, but their entries in the offsets vector are never set. Perl sets its  counted, but their entries in the offsets vector are never set. Perl sets its
# 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) A pattern such as (?|(?<a>A)|(?<b)B), where the two capturing  .P
106  parentheses have the same number but different names, is not supported, and  12. Perl recognizes comments in some places that PCRE doesn't, for example,
107  causes an error at compile time. If it were allowed, it would not be possible  between the ( and ? at the start of a subpattern.
 to distinguish which parentheses matched, because both names map to capturing  
 subpattern number 1. To avoid this confusing situation, an error is given at  
 compile time.  
 .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 172  Cambridge CB2 3QH, England. Line 163  Cambridge CB2 3QH, England.
163  .rs  .rs
164  .sp  .sp
165  .nf  .nf
166  Last updated: 03 October 2009  Last updated: 02 May 2011
167  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
168  .fi  .fi
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