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 with respect to 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&. PCRE does support the Cs (surrogate) property, which Perl does not; the
Perl documentation says "Because Perl hides the need for the user to understand
the internal representation of Unicode characters, there is no need to
implement the somewhat messy concept of surrogates."
.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$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
.P
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.P
9. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl. There 
is a discussion of an example that explains this in more detail in the
.\" HTML <a href="pcrepattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcrepattern\fP
.\"
page.
.P
10. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ 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).
.P
12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern 
names is not as general as Perl's. This is a consequence of the fact the PCRE 
works internally just with numbers, using an external table to translate 
between numbers and names. The following are some specific differences:
.sp
(a) After matching a pattern such as (?|(?<a>A)|(?<b)B) where the two capturing 
parentheses have the same number but different names, it is not possible to 
distinguish which parentheses matched, because both names map to capturing
subpattern number 1.
.sp
(b) A condition test for a subpattern with a name that is duplicated gives
unpredictable results. For example, when the pattern
(?:(?<a>A)|(?<a>B))(?('a')...|...) is compiled (the PCRE_DUPNAMES option is
required), the condition test (?('a') is set to test whether subpattern 1 has
matched, ignoring subpattern 2, even though it has the same name.
.P
13. PCRE provides some extensions to the Perl regular expression facilities.
Perl 5.10 includes new features that are not in earlier versions of Perl, 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 in PCRE 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, PCRE_NOTEMPTY_ATSTART, and
PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
.sp
(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
by the PCRE_BSR_ANYCRLF option.
.sp
(h) The callout facility is PCRE-specific.
.sp
(i) The partial matching facility is PCRE-specific.
.sp
(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
different hosts that have the other endianness.
.sp
(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
different way and is not Perl-compatible.
.sp
(l) PCRE recognizes some special sequences such as (*CR) at the start of
a pattern that set overall options that cannot be changed within the pattern.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 29 September 2009
Copyright (c) 1997-2009 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	ph10	456	regular expressions. The differences described here are with respect to Perl
9			5.10.
10	nigel	75	.P
11	nigel	87	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
13	nigel	73	.\" HTML <a href="pcre.html#utf8support">
14			.\" </a>
15			section on UTF-8 support
16			.\"
17			in the main
18			.\" HREF
19	nigel	75	\fBpcre\fP
20	nigel	73	.\"
21			page.
22	nigel	75	.P
23	nigel	73	2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
24	nigel	63	them, but they do not mean what you might think. For example, (?!a){3} does
25			not assert that the next three characters are not "a". It just asserts that the
26			next character is not "a" three times.
27	nigel	75	.P
28	nigel	73	3. Capturing subpatterns that occur inside negative lookahead assertions are
29	nigel	63	counted, but their entries in the offsets vector are never set. Perl sets its
30			numerical variables from any such patterns that are matched before the
31			assertion fails to match something (thereby succeeding), but only if the
32			negative lookahead assertion contains just one branch.
33	nigel	75	.P
34	nigel	73	4. Though binary zero characters are supported in the subject string, they are
35	nigel	63	not allowed in a pattern string because it is passed as a normal C string,
36	nigel	75	terminated by zero. The escape sequence \e0 can be used in the pattern to
37	nigel	63	represent a binary zero.
38	nigel	75	.P
39			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
41			and are not part of its pattern matching engine. If any of these are
42			encountered by PCRE, an error is generated.
43			.P
44			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
46			tested with \ep and \eP are limited to the general category properties such as
47	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
48	ph10	450	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	nigel	75	.P
53			7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
54	nigel	63	between are treated as literals. This is slightly different from Perl in that $
55			and @ are also handled as literals inside the quotes. In Perl, they cause
56			variable interpolation (but of course PCRE does not have variables). Note the
57			following examples:
58	nigel	75	.sp
59	nigel	63	Pattern PCRE matches Perl matches
60	nigel	75	.sp
61			.\" JOIN
62			\eQabc$xyz\eE abc$xyz abc followed by the
63	nigel	63	contents of $xyz
64	nigel	75	\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
65			\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
66			.sp
67			The \eQ...\eE sequence is recognized both inside and outside character classes.
68			.P
69	nigel	93	8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70			constructions. However, there is support for recursive patterns. This is not
71	ph10	453	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72	nigel	93	feature allows an external function to be called during pattern matching. See
73			the
74	nigel	75	.\" HREF
75			\fBpcrecallout\fP
76			.\"
77			documentation for details.
78			.P
79	nigel	93	9. Subpatterns that are called recursively or as "subroutines" are always
80	ph10	453	treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81			is a discussion of an example that explains this in more detail in the
82			.\" HTML <a href="pcrepattern.html#recursiondifference">
83			.\" </a>
84			section on recursion differences from Perl
85			.\"
86			in the
87			.\" HREF
88	ph10	456	\fBpcrepattern\fP
89	ph10	453	.\"
90			page.
91	nigel	93	.P
92			10. There are some differences that are concerned with the settings of captured
93	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
94			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
95	nigel	75	.P
96	ph10	210	11. PCRE does support Perl 5.10's backtracking verbs (ACCEPT), (FAIL), (*F),
97			(COMMIT), (PRUNE), (SKIP), and (THEN), but only in the forms without an
98	ph10	447	argument. PCRE does not support (*MARK).
99	ph10	210	.P
100	ph10	456	12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
101			names is not as general as Perl's. This is a consequence of the fact the PCRE
102			works internally just with numbers, using an external table to translate
103			between numbers and names. The following are some specific differences:
104	nigel	75	.sp
105	ph10	456	(a) After matching a pattern such as (?\|(?<a>A)\|(?<b)B) where the two capturing
106			parentheses have the same number but different names, it is not possible to
107			distinguish which parentheses matched, because both names map to capturing
108			subpattern number 1.
109	nigel	75	.sp
110	ph10	456	(b) A condition test for a subpattern with a name that is duplicated gives
111			unpredictable results. For example, when the pattern
112			(?:(?<a>A)\|(?<a>B))(?('a')...\|...) is compiled (the PCRE_DUPNAMES option is
113			required), the condition test (?('a') is set to test whether subpattern 1 has
114			matched, ignoring subpattern 2, even though it has the same name.
115			.P
116			13. PCRE provides some extensions to the Perl regular expression facilities.
117			Perl 5.10 includes new features that are not in earlier versions of Perl, some
118			of which (such as named parentheses) have been in PCRE for some time. This list
119			is with respect to Perl 5.10:
120			.sp
121			(a) Although lookbehind assertions in PCRE must match fixed length strings,
122			each alternative branch of a lookbehind assertion can match a different length
123			of string. Perl requires them all to have the same length.
124			.sp
125	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
126			meta-character matches only at the very end of the string.
127	nigel	75	.sp
128	nigel	63	(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
129	ph10	181	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
130			(Perl can be made to issue a warning.)
131	nigel	75	.sp
132	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
133			inverted, that is, by default they are not greedy, but if followed by a
134			question mark they are.
135	nigel	75	.sp
136			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
137			only at the first matching position in the subject string.
138			.sp
139	ph10	442	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
140			PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
141	nigel	75	.sp
142	ph10	231	(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
143			by the PCRE_BSR_ANYCRLF option.
144	nigel	75	.sp
145	ph10	231	(h) The callout facility is PCRE-specific.
146	nigel	75	.sp
147	ph10	231	(i) The partial matching facility is PCRE-specific.
148			.sp
149			(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
150	nigel	75	different hosts that have the other endianness.
151	nigel	77	.sp
152	ph10	231	(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
153	nigel	77	different way and is not Perl-compatible.
154	ph10	231	.sp
155			(l) PCRE recognizes some special sequences such as (*CR) at the start of
156			a pattern that set overall options that cannot be changed within the pattern.
157	ph10	99	.
158			.
159			.SH AUTHOR
160			.rs
161			.sp
162			.nf
163			Philip Hazel
164			University Computing Service
165			Cambridge CB2 3QH, England.
166			.fi
167			.
168			.
169			.SH REVISION
170			.rs
171			.sp
172			.nf
173	ph10	456	Last updated: 29 September 2009
174	ph10	426	Copyright (c) 1997-2009 University of Cambridge.
175	ph10	99	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"