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revision 457 by ph10, Sat Oct 3 16:24:08 2009 UTC revision 635 by ph10, Sat Jul 23 16:19:50 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 37  terminated by zero. The escape sequence Line 37  terminated by zero. The escape sequence
37  represent a binary zero.  represent a binary zero.
38  .P  .P
39  5. The following Perl escape sequences are not supported: \el, \eu, \eL,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
40  \eU, and \eN. In fact these are implemented by Perl's general string-handling  \eU, and \eN when followed by a character name or Unicode value. (\eN on its
41  and are not part of its pattern matching engine. If any of these are  own, matching a non-newline character, is supported.) In fact these are
42  encountered by PCRE, an error is generated.  implemented by Perl's general string-handling and are not part of its pattern
43    matching engine. If any of these are encountered by PCRE, an error is
44    generated.
45  .P  .P
46  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
47  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
# Line 50  Perl documentation says "Because Perl hi Line 52  Perl documentation says "Because Perl hi
52  the internal representation of Unicode characters, there is no need to  the internal representation of Unicode characters, there is no need to
53  implement the somewhat messy concept of surrogates."  implement the somewhat messy concept of surrogates."
54  .P  .P
55  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in  7. PCRE implements a simpler version of \eX than Perl, which changed to make
56    \eX match what Unicode calls an "extended grapheme cluster". This is more
57    complicated than an extended Unicode sequence, which is what PCRE matches.
58    .P
59    8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
60  between are treated as literals. This is slightly different from Perl in that $  between are treated as literals. This is slightly different from Perl in that $
61  and @ are also handled as literals inside the quotes. In Perl, they cause  and @ are also handled as literals inside the quotes. In Perl, they cause
62  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
# Line 66  following examples: Line 72  following examples:
72  .sp  .sp
73  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
74  .P  .P
75  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
76  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
77  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
78  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
# Line 76  the Line 82  the
82  .\"  .\"
83  documentation for details.  documentation for details.
84  .P  .P
85  9. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called recursively or as "subroutines" are always
86  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
87  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
88  .\" HTML <a href="pcrepattern.html#recursiondifference">  .\" HTML <a href="pcrepattern.html#recursiondifference">
89  .\" </a>  .\" </a>
# Line 89  in the Line 95  in the
95  .\"  .\"
96  page.  page.
97  .P  .P
98  10. There are some differences that are concerned with the settings of captured  11. There are some differences that are concerned with the settings of captured
99  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
100  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".
101  .P  .P
102  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
103  (*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
104  argument. PCRE does not support (*MARK).  works internally just with numbers, using an external table to translate
105  .P  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
106  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern  where the two capturing parentheses have the same number but different names,
107  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
108  works internally just with numbers, using an external table to translate  would not be possible to distinguish which parentheses matched, because both
109  between numbers and names. The following are some specific differences:  names map to capturing subpattern number 1. To avoid this confusing situation,
110  .sp  an error is given at compile time.
111  (a) A pattern such as (?|(?<a>A)|(?<b)B), where the two capturing  .P
112  parentheses have the same number but different names, is not supported, and  13. Perl recognizes comments in some places that PCRE does not, for example,
113  causes an error at compile time. If it were allowed, it would not be possible  between the ( and ? at the start of a subpattern. If the /x modifier is set,
114  to distinguish which parentheses matched, because both names map to capturing  Perl allows whitespace between ( and ? but PCRE never does, even if the
115  subpattern number 1. To avoid this confusing situation, an error is given at  PCRE_EXTENDED option is set.
 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.  
116  .P  .P
117  13. PCRE provides some extensions to the Perl regular expression facilities.  14. PCRE provides some extensions to the Perl regular expression facilities.
118  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
119  of which (such as named parentheses) have been in PCRE for some time. This list  of which (such as named parentheses) have been in PCRE for some time. This list
120  is with respect to Perl 5.10:  is with respect to Perl 5.10:
# Line 172  Cambridge CB2 3QH, England. Line 171  Cambridge CB2 3QH, England.
171  .rs  .rs
172  .sp  .sp
173  .nf  .nf
174  Last updated: 03 October 2009  Last updated: 23 July 2011
175  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
176  .fi  .fi
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