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revision 459 by ph10, Sun Oct 4 09:21:39 2009 UTC revision 678 by ph10, Sun Aug 28 15:23:03 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
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
13  .\" HREF  .\" HREF
14  \fBpcre\fP  \fBpcreunicode\fP
15  .\"  .\"
16  page.  page.
17  .P  .P
18  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19  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
20  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
21  next character is not "a" three times.  not "a" three times (in principle: PCRE optimizes this to run the assertion
22    just once). Perl allows repeat quantifiers on other assertions such as \eb, but
23    these do not seem to have any use.
24  .P  .P
25  3. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
26  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 34  terminated by zero. The escape sequence
34  represent a binary zero.  represent a binary zero.
35  .P  .P
36  5. The following Perl escape sequences are not supported: \el, \eu, \eL,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37  \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
38  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
39  encountered by PCRE, an error is generated.  implemented by Perl's general string-handling and are not part of its pattern
40    matching engine. If any of these are encountered by PCRE, an error is
41    generated.
42  .P  .P
43  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
44  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 49  Perl documentation says "Because Perl hi
49  the internal representation of Unicode characters, there is no need to  the internal representation of Unicode characters, there is no need to
50  implement the somewhat messy concept of surrogates."  implement the somewhat messy concept of surrogates."
51  .P  .P
52  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
53    \eX match what Unicode calls an "extended grapheme cluster". This is more
54    complicated than an extended Unicode sequence, which is what PCRE matches.
55    .P
56    8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
57  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 $
58  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
59  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 69  following examples:
69  .sp  .sp
70  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
71  .P  .P
72  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
73  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
74  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"
75  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 79  the
79  .\"  .\"
80  documentation for details.  documentation for details.
81  .P  .P
82  9. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called recursively or as "subroutines" are always
83  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
84  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
85  .\" HTML <a href="pcrepattern.html#recursiondifference">  .\" HTML <a href="pcrepattern.html#recursiondifference">
86  .\" </a>  .\" </a>
# Line 89  in the Line 92  in the
92  .\"  .\"
93  page.  page.
94  .P  .P
95  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
96  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
97  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".
98  .P  .P
99  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
 (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an  
 argument. PCRE does not support (*MARK).  
 .P  
 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern  
100  names is not as general as Perl's. This is a consequence of the fact the PCRE  names is not as general as Perl's. This is a consequence of the fact the PCRE
101  works internally just with numbers, using an external table to translate  works internally just with numbers, using an external table to translate
102  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
# Line 107  would not be possible to distinguish whi Line 106  would not be possible to distinguish whi
106  names map to capturing subpattern number 1. To avoid this confusing situation,  names map to capturing subpattern number 1. To avoid this confusing situation,
107  an error is given at compile time.  an error is given at compile time.
108  .P  .P
109  13. PCRE provides some extensions to the Perl regular expression facilities.  13. Perl recognizes comments in some places that PCRE does not, for example,
110    between the ( and ? at the start of a subpattern. If the /x modifier is set,
111    Perl allows whitespace between ( and ? but PCRE never does, even if the
112    PCRE_EXTENDED option is set.
113    .P
114    14. PCRE provides some extensions to the Perl regular expression facilities.
115  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
116  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
117  is with respect to Perl 5.10:  is with respect to Perl 5.10:
# Line 164  Cambridge CB2 3QH, England. Line 168  Cambridge CB2 3QH, England.
168  .rs  .rs
169  .sp  .sp
170  .nf  .nf
171  Last updated: 04 October 2009  Last updated: 24 August 2011
172  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
173  .fi  .fi

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