trunk/doc/pcrecompat.3

.TH PCRECOMPAT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "DIFFERENCES BETWEEN PCRE AND PERL"
.rs
.sp
This document describes the differences in the ways that PCRE and Perl handle
regular expressions. The differences described here are mainly with respect to
Perl 5.8, though PCRE versions 7.0 and later contain some features that are
expected to be in the forthcoming Perl 5.10.
.P
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
it does have are given in the
.\" HTML <a href="pcre.html#utf8support">
.\" </a>
section on UTF-8 support
.\"
in the main
.\" HREF
\fBpcre\fP
.\"
page.
.P
2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
them, but they do not mean what you might think. For example, (?!a){3} does
not assert that the next three characters are not "a". It just asserts that the
next character is not "a" three times.
.P
3. Capturing subpatterns that occur inside negative lookahead assertions are
counted, but their entries in the offsets vector are never set. Perl sets its
numerical variables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but only if the
negative lookahead assertion contains just one branch.
.P
4. Though binary zero characters are supported in the subject string, they are
not allowed in a pattern string because it is passed as a normal C string,
terminated by zero. The escape sequence \e0 can be used in the pattern to
represent a binary zero.
.P
5. The following Perl escape sequences are not supported: \el, \eu, \eL,
\eU, and \eN. In fact these are implemented by Perl's general string-handling
and are not part of its pattern matching engine. If any of these are
encountered by PCRE, an error is generated.
.P
6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
built with Unicode character property support. The properties that can be
tested with \ep and \eP are limited to the general category properties such as
Lu and Nd, script names such as Greek or Han, and the derived properties Any
and L&.
.P
7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
between are treated as literals. This is slightly different from Perl in that $
and @ are also handled as literals inside the quotes. In Perl, they cause
variable interpolation (but of course PCRE does not have variables). Note the
following examples:
.sp
    Pattern            PCRE matches      Perl matches
.sp
.\" JOIN
    \eQabc$xyz\eE        abc$xyz           abc followed by the
                                           contents of $xyz
    \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
    \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
.P
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.P
9. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl.
.P
10. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
.P
11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing 
parentheses, PCRE does not set that capture group; this is different to Perl.
.P
12. PCRE provides some extensions to the Perl regular expression facilities.
Perl 5.10 will include new features that are not in earlier versions, some of
which (such as named parentheses) have been in PCRE for some time. This list is
with respect to Perl 5.10:
.sp
(a) Although lookbehind assertions must match fixed length strings, each
alternative branch of a lookbehind assertion can match a different length of
string. Perl requires them all to have the same length.
.sp
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
meta-character matches only at the very end of the string.
.sp
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
(Perl can be made to issue a warning.)
.sp
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
inverted, that is, by default they are not greedy, but if followed by a
question mark they are.
.sp
(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
only at the first matching position in the subject string.
.sp
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
options for \fBpcre_exec()\fP have no Perl equivalents.
.sp
(g) The callout facility is PCRE-specific.
.sp
(h) The partial matching facility is PCRE-specific.
.sp
(i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
different hosts that have the other endianness.
.sp
(j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
different way and is not Perl-compatible.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 08 August 2007
Copyright (c) 1997-2007 University of Cambridge.
.fi
1	nigel	79	.TH PCRECOMPAT 3
2	nigel	63	.SH NAME
3			PCRE - Perl-compatible regular expressions
4	nigel	75	.SH "DIFFERENCES BETWEEN PCRE AND PERL"
5	nigel	63	.rs
6			.sp
7			This document describes the differences in the ways that PCRE and Perl handle
8	nigel	93	regular expressions. The differences described here are mainly with respect to
9	ph10	181	Perl 5.8, though PCRE versions 7.0 and later contain some features that are
10			expected to be in the forthcoming Perl 5.10.
11	nigel	75	.P
12	nigel	87	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	nigel	73	.\" HTML <a href="pcre.html#utf8support">
15			.\" </a>
16			section on UTF-8 support
17			.\"
18			in the main
19			.\" HREF
20	nigel	75	\fBpcre\fP
21	nigel	73	.\"
22			page.
23	nigel	75	.P
24	nigel	73	2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
25	nigel	63	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
27			next character is not "a" three times.
28	nigel	75	.P
29	nigel	73	3. Capturing subpatterns that occur inside negative lookahead assertions are
30	nigel	63	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
32			assertion fails to match something (thereby succeeding), but only if the
33			negative lookahead assertion contains just one branch.
34	nigel	75	.P
35	nigel	73	4. Though binary zero characters are supported in the subject string, they are
36	nigel	63	not allowed in a pattern string because it is passed as a normal C string,
37	nigel	75	terminated by zero. The escape sequence \e0 can be used in the pattern to
38	nigel	63	represent a binary zero.
39	nigel	75	.P
40			5. The following Perl escape sequences are not supported: \el, \eu, \eL,
41			\eU, and \eN. In fact these are implemented by Perl's general string-handling
42			and are not part of its pattern matching engine. If any of these are
43			encountered by PCRE, an error is generated.
44			.P
45			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	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
49			and L&.
50	nigel	75	.P
51			7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
52	nigel	63	between are treated as literals. This is slightly different from Perl in that $
53			and @ are also handled as literals inside the quotes. In Perl, they cause
54			variable interpolation (but of course PCRE does not have variables). Note the
55			following examples:
56	nigel	75	.sp
57	nigel	63	Pattern PCRE matches Perl matches
58	nigel	75	.sp
59			.\" JOIN
60			\eQabc$xyz\eE abc$xyz abc followed by the
61	nigel	63	contents of $xyz
62	nigel	75	\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
63			\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
64			.sp
65			The \eQ...\eE sequence is recognized both inside and outside character classes.
66			.P
67	nigel	93	8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
68			constructions. However, there is support for recursive patterns. This is not
69			available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
70			feature allows an external function to be called during pattern matching. See
71			the
72	nigel	75	.\" HREF
73			\fBpcrecallout\fP
74			.\"
75			documentation for details.
76			.P
77	nigel	93	9. Subpatterns that are called recursively or as "subroutines" are always
78			treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79			.P
80			10. There are some differences that are concerned with the settings of captured
81	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
82			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
83	nigel	75	.P
84	ph10	210	11. PCRE does support Perl 5.10's backtracking verbs (ACCEPT), (FAIL), (*F),
85			(COMMIT), (PRUNE), (SKIP), and (THEN), but only in the forms without an
86			argument. PCRE does not support (MARK). If (ACCEPT) is within capturing
87			parentheses, PCRE does not set that capture group; this is different to Perl.
88			.P
89			12. PCRE provides some extensions to the Perl regular expression facilities.
90	nigel	93	Perl 5.10 will include new features that are not in earlier versions, some of
91			which (such as named parentheses) have been in PCRE for some time. This list is
92			with respect to Perl 5.10:
93	nigel	75	.sp
94	nigel	63	(a) Although lookbehind assertions must match fixed length strings, each
95			alternative branch of a lookbehind assertion can match a different length of
96			string. Perl requires them all to have the same length.
97	nigel	75	.sp
98	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
99			meta-character matches only at the very end of the string.
100	nigel	75	.sp
101	nigel	63	(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
102	ph10	181	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
103			(Perl can be made to issue a warning.)
104	nigel	75	.sp
105	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
106			inverted, that is, by default they are not greedy, but if followed by a
107			question mark they are.
108	nigel	75	.sp
109			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
110			only at the first matching position in the subject string.
111			.sp
112	nigel	63	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
113	nigel	75	options for \fBpcre_exec()\fP have no Perl equivalents.
114			.sp
115	nigel	93	(g) The callout facility is PCRE-specific.
116	nigel	75	.sp
117	nigel	93	(h) The partial matching facility is PCRE-specific.
118	nigel	75	.sp
119	nigel	93	(i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
120	nigel	75	different hosts that have the other endianness.
121	nigel	77	.sp
122	nigel	93	(j) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
123	nigel	77	different way and is not Perl-compatible.
124	ph10	99	.
125			.
126			.SH AUTHOR
127			.rs
128			.sp
129			.nf
130			Philip Hazel
131			University Computing Service
132			Cambridge CB2 3QH, England.
133			.fi
134			.
135			.
136			.SH REVISION
137			.rs
138			.sp
139			.nf
140	ph10	210	Last updated: 08 August 2007
141	ph10	99	Copyright (c) 1997-2007 University of Cambridge.
142			.fi
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svn:keywords	"Author Date Id Revision Url"