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revision 442 by ph10, Fri Sep 11 10:21:02 2009 UTC revision 637 by ph10, Sun Jul 24 17:44:12 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.
 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 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 allows repeat quantifiers only on parenthesized assertions, but they do
24  them, but they do not mean what you might think. For example, (?!a){3} does  not mean what you might think. For example, (?!a){3} does not assert that the
25  not assert that the next three characters are not "a". It just asserts that the  next three characters are not "a". It just asserts that the next character is
26  next character is not "a" three times.  not "a" three times (in principle: PCRE optimizes this to run the assertion
27    just once). Perl allows repeat quantifiers on other assertions such as \b, but
28    these do not seem to have any use.
29  .P  .P
30  3. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
31  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 38  terminated by zero. The escape sequence Line 39  terminated by zero. The escape sequence
39  represent a binary zero.  represent a binary zero.
40  .P  .P
41  5. The following Perl escape sequences are not supported: \el, \eu, \eL,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
42  \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
43  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
44  encountered by PCRE, an error is generated.  implemented by Perl's general string-handling and are not part of its pattern
45    matching engine. If any of these are encountered by PCRE, an error is
46    generated.
47  .P  .P
48  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
49  built with Unicode character property support. The properties that can be  built with Unicode character property support. The properties that can be
50  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
51  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
52  and L&.  and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
53    Perl documentation says "Because Perl hides the need for the user to understand
54    the internal representation of Unicode characters, there is no need to
55    implement the somewhat messy concept of surrogates."
56    .P
57    7. PCRE implements a simpler version of \eX than Perl, which changed to make
58    \eX match what Unicode calls an "extended grapheme cluster". This is more
59    complicated than an extended Unicode sequence, which is what PCRE matches.
60  .P  .P
61  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in  8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
62  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 $
63  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
64  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 64  following examples: Line 74  following examples:
74  .sp  .sp
75  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
76  .P  .P
77  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
78  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
79  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"
80  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
81  the  the
82  .\" HREF  .\" HREF
# Line 74  the Line 84  the
84  .\"  .\"
85  documentation for details.  documentation for details.
86  .P  .P
87  9. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called recursively or as "subroutines" are always
88  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
89    is a discussion of an example that explains this in more detail in the
90    .\" HTML <a href="pcrepattern.html#recursiondifference">
91    .\" </a>
92    section on recursion differences from Perl
93    .\"
94    in the
95    .\" HREF
96    \fBpcrepattern\fP
97    .\"
98    page.
99  .P  .P
100  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
101  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
102  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".
103  .P  .P
104  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
105  (*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
106  argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing  works internally just with numbers, using an external table to translate
107  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),
108  .P  where the two capturing parentheses have the same number but different names,
109  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
110  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
111  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,
112  with respect to Perl 5.10:  an error is given at compile time.
113  .sp  .P
114  (a) Although lookbehind assertions must match fixed length strings, each  13. Perl recognizes comments in some places that PCRE does not, for example,
115  alternative branch of a lookbehind assertion can match a different length of  between the ( and ? at the start of a subpattern. If the /x modifier is set,
116  string. Perl requires them all to have the same length.  Perl allows whitespace between ( and ? but PCRE never does, even if the
117    PCRE_EXTENDED option is set.
118    .P
119    14. PCRE provides some extensions to the Perl regular expression facilities.
120    Perl 5.10 includes new features that are not in earlier versions of Perl, some
121    of which (such as named parentheses) have been in PCRE for some time. This list
122    is with respect to Perl 5.10:
123    .sp
124    (a) Although lookbehind assertions in PCRE must match fixed length strings,
125    each alternative branch of a lookbehind assertion can match a different length
126    of string. Perl requires them all to have the same length.
127  .sp  .sp
128  (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 $
129  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
# Line 143  Cambridge CB2 3QH, England. Line 173  Cambridge CB2 3QH, England.
173  .rs  .rs
174  .sp  .sp
175  .nf  .nf
176  Last updated: 11 September 2009  Last updated: 24 July 2011
177  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
178  .fi  .fi

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