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revision 63 by nigel, Sat Feb 24 21:40:03 2007 UTC revision 450 by ph10, Wed Sep 16 10:56:40 2009 UTC
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1  .TH PCRE 3  .TH PCRECOMPAT 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH DIFFERENCES FROM PERL  .SH "DIFFERENCES BETWEEN PCRE AND PERL"
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 with respect to Perl  regular expressions. The differences described here are mainly with respect to
9  5.8.  Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10    in Perl 5.10.
11  1. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  .P
12    1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
13    it does have are given in the
14    .\" HTML <a href="pcre.html#utf8support">
15    .\" </a>
16    section on UTF-8 support
17    .\"
18    in the main
19    .\" HREF
20    \fBpcre\fP
21    .\"
22    page.
23    .P
24    2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
25  them, but they do not mean what you might think. For example, (?!a){3} does  them, but they do not mean what you might think. For example, (?!a){3} does
26  not assert that the next three characters are not "a". It just asserts that the  not assert that the next three characters are not "a". It just asserts that the
27  next character is not "a" three times.  next character is not "a" three times.
28    .P
29  2. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
30  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
31  numerical variables from any such patterns that are matched before the  numerical variables from any such patterns that are matched before the
32  assertion fails to match something (thereby succeeding), but only if the  assertion fails to match something (thereby succeeding), but only if the
33  negative lookahead assertion contains just one branch.  negative lookahead assertion contains just one branch.
34    .P
35  3. Though binary zero characters are supported in the subject string, they are  4. Though binary zero characters are supported in the subject string, they are
36  not allowed in a pattern string because it is passed as a normal C string,  not allowed in a pattern string because it is passed as a normal C string,
37  terminated by zero. The escape sequence "\\0" can be used in the pattern to  terminated by zero. The escape sequence \e0 can be used in the pattern to
38  represent a binary zero.  represent a binary zero.
39    .P
40  4. The following Perl escape sequences are not supported: \\l, \\u, \\L,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
41  \\U, \\P, \\p, \N, and \\X. In fact these are implemented by Perl's general  \eU, and \eN. In fact these are implemented by Perl's general string-handling
42  string-handling and are not part of its pattern matching engine. If any of  and are not part of its pattern matching engine. If any of these are
43  these are encountered by PCRE, an error is generated.  encountered by PCRE, an error is generated.
44    .P
45  5. PCRE does support the \\Q...\\E escape for quoting substrings. Characters in  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
46    built with Unicode character property support. The properties that can be
47    tested with \ep and \eP are limited to the general category properties such as
48    Lu and Nd, script names such as Greek or Han, and the derived properties Any
49    and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
50    Perl documentation says "Because Perl hides the need for the user to understand
51    the internal representation of Unicode characters, there is no need to
52    implement the somewhat messy concept of surrogates."
53    .P
54    7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
55  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 $
56  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
57  variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
58  following examples:  following examples:
59    .sp
60      Pattern            PCRE matches      Perl matches      Pattern            PCRE matches      Perl matches
61    .sp
62      \\Qabc$xyz\\E        abc$xyz           abc followed by the  .\" JOIN
63        \eQabc$xyz\eE        abc$xyz           abc followed by the
64                                             contents of $xyz                                             contents of $xyz
65      \\Qabc\\$xyz\\E       abc\\$xyz          abc\\$xyz      \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
66      \\Qabc\\E\\$\\Qxyz\\E   abc$xyz           abc$xyz      \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
67    .sp
68  In PCRE, the \\Q...\\E mechanism is not recognized inside a character class.  The \eQ...\eE sequence is recognized both inside and outside character classes.
69    .P
70  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
71  constructions. However, there is some experimental support for recursive  constructions. However, there is support for recursive patterns. This is not
72  patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE  available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
73  "callout" feature allows an external function to be called during pattern  feature allows an external function to be called during pattern matching. See
74  matching.  the
75    .\" HREF
76  9. There are some differences that are concerned with the settings of captured  \fBpcrecallout\fP
77    .\"
78    documentation for details.
79    .P
80    9. Subpatterns that are called recursively or as "subroutines" are always
81    treated as atomic groups in PCRE. This is like Python, but unlike Perl.
82    .P
83    10. There are some differences that are concerned with the settings of captured
84  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
85  the pattern /^(a(b)?)+$/ 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".
86    .P
87  10. PCRE provides some extensions to the Perl regular expression facilities:  11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
88    (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
89    argument. PCRE does not support (*MARK).
90    .P
91    12. PCRE provides some extensions to the Perl regular expression facilities.
92    Perl 5.10 will include new features that are not in earlier versions, some of
93    which (such as named parentheses) have been in PCRE for some time. This list is
94    with respect to Perl 5.10:
95    .sp
96  (a) Although lookbehind assertions must match fixed length strings, each  (a) Although lookbehind assertions must match fixed length strings, each
97  alternative branch of a lookbehind assertion can match a different length of  alternative branch of a lookbehind assertion can match a different length of
98  string. Perl requires them all to have the same length.  string. Perl requires them all to have the same length.
99    .sp
100  (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 $
101  meta-character matches only at the very end of the string.  meta-character matches only at the very end of the string.
102    .sp
103  (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
104  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
105    (Perl can be made to issue a warning.)
106    .sp
107  (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
108  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
109  question mark they are.  question mark they are.
110    .sp
111  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
112  matching position in the subject string.  only at the first matching position in the subject string.
113    .sp
114  (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
115  options for \fBpcre_exec()\fR have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
116    .sp
117  (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
118  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
119  support.)  .sp
120    (h) The callout facility is PCRE-specific.
121  (h) PCRE supports named capturing substrings, using the Python syntax.  .sp
122    (i) The partial matching facility is PCRE-specific.
123  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  .sp
124  package.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
125    different hosts that have the other endianness.
126  (j) The (R) condition, for testing recursion, is a PCRE extension.  .sp
127    (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
128  (k) The callout facility is PCRE-specific.  different way and is not Perl-compatible.
129    .sp
130  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
131  Last updated: 03 February 2003  a pattern that set overall options that cannot be changed within the pattern.
132  .br  .
133  Copyright (c) 1997-2003 University of Cambridge.  .
134    .SH AUTHOR
135    .rs
136    .sp
137    .nf
138    Philip Hazel
139    University Computing Service
140    Cambridge CB2 3QH, England.
141    .fi
142    .
143    .
144    .SH REVISION
145    .rs
146    .sp
147    .nf
148    Last updated: 16 September 2009
149    Copyright (c) 1997-2009 University of Cambridge.
150    .fi
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