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revision 99 by ph10, Tue Mar 6 12:27:42 2007 UTC revision 633 by ph10, Sat Jul 23 14:34:27 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 version 7.0 contains some features that are expected to  versions 5.10 and above.
 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 38  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  and \eU. In fact these are implemented by Perl's general string-handling and
41  and are not part of its pattern matching engine. If any of these are  are not part of its pattern matching engine. If any of these are encountered by
42  encountered by PCRE, an error is generated.  PCRE, an error is generated.
43  .P  .P
44  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
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
53    7. PCRE implements a simpler version of \eX than Perl, which changed to make
54    \eX match what Unicode calls an "extended grapheme cluster". This is more
55    complicated than an extended Unicode sequence, which is what PCRE matches.
56  .P  .P
57  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
58  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 $
59  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
60  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 70  following examples:
70  .sp  .sp
71  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
72  .P  .P
73  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
74  constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
75  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"
76  feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
77  the  the
78  .\" HREF  .\" HREF
# Line 74  the Line 80  the
80  .\"  .\"
81  documentation for details.  documentation for details.
82  .P  .P
83  9. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called recursively or as "subroutines" are always
84  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
85    is a discussion of an example that explains this in more detail in the
86    .\" HTML <a href="pcrepattern.html#recursiondifference">
87    .\" </a>
88    section on recursion differences from Perl
89    .\"
90    in the
91    .\" HREF
92    \fBpcrepattern\fP
93    .\"
94    page.
95  .P  .P
96  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
97  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
98  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".
99  .P  .P
100  11. PCRE provides some extensions to the Perl regular expression facilities.  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101  Perl 5.10 will include new features that are not in earlier versions, some of  names is not as general as Perl's. This is a consequence of the fact the PCRE
102  which (such as named parentheses) have been in PCRE for some time. This list is  works internally just with numbers, using an external table to translate
103  with respect to Perl 5.10:  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
104  .sp  where the two capturing parentheses have the same number but different names,
105  (a) Although lookbehind assertions must match fixed length strings, each  is not supported, and causes an error at compile time. If it were allowed, it
106  alternative branch of a lookbehind assertion can match a different length of  would not be possible to distinguish which parentheses matched, because both
107  string. Perl requires them all to have the same length.  names map to capturing subpattern number 1. To avoid this confusing situation,
108    an error is given at compile time.
109    .P
110    13. Perl recognizes comments in some places that PCRE doesn't, for example,
111    between the ( and ? at the start of a subpattern.
112    .P
113    14. PCRE provides some extensions to the Perl regular expression facilities.
114    Perl 5.10 includes new features that are not in earlier versions of Perl, some
115    of which (such as named parentheses) have been in PCRE for some time. This list
116    is with respect to Perl 5.10:
117    .sp
118    (a) Although lookbehind assertions in PCRE must match fixed length strings,
119    each alternative branch of a lookbehind assertion can match a different length
120    of string. Perl requires them all to have the same length.
121  .sp  .sp
122  (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 $
123  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
124  .sp  .sp
125  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
126  meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
127  be made to issue a warning.)  (Perl can be made to issue a warning.)
128  .sp  .sp
129  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
130  inverted, that is, by default they are not greedy, but if followed by a  inverted, that is, by default they are not greedy, but if followed by a
# Line 104  question mark they are. Line 133  question mark they are.
133  (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
134  only at the first matching position in the subject string.  only at the first matching position in the subject string.
135  .sp  .sp
136  (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
137  options for \fBpcre_exec()\fP have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
138    .sp
139    (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
140    by the PCRE_BSR_ANYCRLF option.
141  .sp  .sp
142  (g) The callout facility is PCRE-specific.  (h) The callout facility is PCRE-specific.
143  .sp  .sp
144  (h) The partial matching facility is PCRE-specific.  (i) The partial matching facility is PCRE-specific.
145  .sp  .sp
146  (i) Patterns compiled by PCRE can be saved and re-used at a later time, even on  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
147  different hosts that have the other endianness.  different hosts that have the other endianness.
148  .sp  .sp
149  (j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
150  different way and is not Perl-compatible.  different way and is not Perl-compatible.
151    .sp
152    (l) PCRE recognizes some special sequences such as (*CR) at the start of
153    a pattern that set overall options that cannot be changed within the pattern.
154  .  .
155  .  .
156  .SH AUTHOR  .SH AUTHOR
# Line 132  Cambridge CB2 3QH, England. Line 167  Cambridge CB2 3QH, England.
167  .rs  .rs
168  .sp  .sp
169  .nf  .nf
170  Last updated: 06 March 2007  Last updated: 23 July 2011
171  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
172  .fi  .fi
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