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revision 231 by ph10, Tue Sep 11 11:15:33 2007 UTC revision 596 by ph10, Mon May 2 10:47:52 2011 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  versions 5.10 and above.
 expected to be in the forthcoming 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 21  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 46  encountered by PCRE, an error is generat Line 45  encountered by PCRE, an error is generat
45  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
46  tested with \ep and \eP are limited to the general category properties such as  tested with \ep and \eP are limited to the general category properties such as
47  Lu and Nd, script names such as Greek or Han, and the derived properties Any  Lu and Nd, script names such as Greek or Han, and the derived properties Any
48  and L&.  and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49    Perl documentation says "Because Perl hides the need for the user to understand
50    the internal representation of Unicode characters, there is no need to
51    implement the somewhat messy concept of surrogates."
52  .P  .P
53  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
54  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 $
# Line 66  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 75  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
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). If (*ACCEPT) is within capturing  works internally just with numbers, using an external table to translate
99  parentheses, PCRE does not set that capture group; this is different to Perl.  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
100  .P  where the two capturing parentheses have the same number but different names,
101  12. PCRE provides some extensions to the Perl regular expression facilities.  is not supported, and causes an error at compile time. If it were allowed, it
102  Perl 5.10 will include new features that are not in earlier versions, some of  would not be possible to distinguish which parentheses matched, because both
103  which (such as named parentheses) have been in PCRE for some time. This list is  names map to capturing subpattern number 1. To avoid this confusing situation,
104  with respect to Perl 5.10:  an error is given at compile time.
105  .sp  .P
106  (a) Although lookbehind assertions must match fixed length strings, each  12. Perl recognizes comments in some places that PCRE doesn't, for example,
107  alternative branch of a lookbehind assertion can match a different length of  between the ( and ? at the start of a subpattern.
108  string. Perl requires them all to have the same length.  .P
109    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
111    of which (such as named parentheses) have been in PCRE for some time. This list
112    is with respect to Perl 5.10:
113    .sp
114    (a) Although lookbehind assertions in PCRE must match fixed length strings,
115    each alternative branch of a lookbehind assertion can match a different length
116    of string. Perl requires them all to have the same length.
117  .sp  .sp
118  (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 $
119  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
# Line 109  question mark they are. Line 129  question mark they are.
129  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
130  only at the first matching position in the subject string.  only at the first matching position in the subject string.
131  .sp  .sp
132  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
133  options for \fBpcre_exec()\fP have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
134  .sp  .sp
135  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
136  by the PCRE_BSR_ANYCRLF option.  by the PCRE_BSR_ANYCRLF option.
# Line 143  Cambridge CB2 3QH, England. Line 163  Cambridge CB2 3QH, England.
163  .rs  .rs
164  .sp  .sp
165  .nf  .nf
166  Last updated: 11 September 2007  Last updated: 02 May 2011
167  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
168  .fi  .fi

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