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
versions 5.10 and above.
.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
.\" HREF
\fBpcreunicode\fP
.\"
page.
.P
2. PCRE allows repeat quantifiers only on parenthesized assertions, 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 (in principle: PCRE optimizes this to run the assertion
just once). Perl allows repeat quantifiers on other assertions such as \eb, but
these do not seem to have any use.
.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 when followed by a character name or Unicode value. (\eN on its
own, matching a non-newline character, is supported.) 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 implements a simpler version of \eX than Perl, which changed to make
\eX match what Unicode calls an "extended grapheme cluster". This is more
complicated than an extended Unicode sequence, which is what PCRE matches.
.P
8. 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$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
.P
9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.P
10. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
is a discussion of an example that explains this in more detail in the
.\" HTML <a href="pcrepattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcrepattern\fP
.\"
page.
.P
11. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
.P
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. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
where the two capturing parentheses have the same number but different names,
is not supported, and causes an error at compile time. If it were allowed, it
would not be possible to distinguish which parentheses matched, because both
names map to capturing subpattern number 1. To avoid this confusing situation,
an error is given at compile time.
.P
13. Perl recognizes comments in some places that PCRE does not, for example,
between the ( and ? at the start of a subpattern. If the /x modifier is set,
Perl allows whitespace between ( and ? but PCRE never does, even if the
PCRE_EXTENDED option is set.
.P
14. 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: 24 August 2011
Copyright (c) 1997-2011 University of Cambridge.
.fi
1	.TH PCRECOMPAT 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "DIFFERENCES BETWEEN PCRE AND PERL"
5	.rs
6	.sp
7	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
9	versions 5.10 and above.
10	.P
11	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	.\" HREF
14	\fBpcreunicode\fP
15	.\"
16	page.
17	.P
18	2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19	not mean what you might think. For example, (?!a){3} does not assert that the
20	next three characters are not "a". It just asserts that the next character is
21	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
25	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
27	numerical variables from any such patterns that are matched before the
28	assertion fails to match something (thereby succeeding), but only if the
29	negative lookahead assertion contains just one branch.
30	.P
31	4. Though binary zero characters are supported in the subject string, they are
32	not allowed in a pattern string because it is passed as a normal C string,
33	terminated by zero. The escape sequence \e0 can be used in the pattern to
34	represent a binary zero.
35	.P
36	5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37	\eU, and \eN when followed by a character name or Unicode value. (\eN on its
38	own, matching a non-newline character, is supported.) In fact these are
39	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
43	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
45	tested with \ep and \eP are limited to the general category properties such as
46	Lu and Nd, script names such as Greek or Han, and the derived properties Any
47	and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
48	Perl documentation says "Because Perl hides the need for the user to understand
49	the internal representation of Unicode characters, there is no need to
50	implement the somewhat messy concept of surrogates."
51	.P
52	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 $
58	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
60	following examples:
61	.sp
62	Pattern PCRE matches Perl matches
63	.sp
64	.\" JOIN
65	\eQabc$xyz\eE abc$xyz abc followed by the
66	contents of $xyz
67	\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
68	\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
69	.sp
70	The \eQ...\eE sequence is recognized both inside and outside character classes.
71	.P
72	9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
73	constructions. However, there is support for recursive patterns. This is not
74	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
75	feature allows an external function to be called during pattern matching. See
76	the
77	.\" HREF
78	\fBpcrecallout\fP
79	.\"
80	documentation for details.
81	.P
82	10. Subpatterns that are called recursively or as "subroutines" are always
83	treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
84	is a discussion of an example that explains this in more detail in the
85	.\" HTML <a href="pcrepattern.html#recursiondifference">
86	.\" </a>
87	section on recursion differences from Perl
88	.\"
89	in the
90	.\" HREF
91	\fBpcrepattern\fP
92	.\"
93	page.
94	.P
95	11. There are some differences that are concerned with the settings of captured
96	strings when part of a pattern is repeated. For example, matching "aba" against
97	the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
98	.P
99	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
101	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),
103	where the two capturing parentheses have the same number but different names,
104	is not supported, and causes an error at compile time. If it were allowed, it
105	would not be possible to distinguish which parentheses matched, because both
106	names map to capturing subpattern number 1. To avoid this confusing situation,
107	an error is given at compile time.
108	.P
109	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
116	of which (such as named parentheses) have been in PCRE for some time. This list
117	is with respect to Perl 5.10:
118	.sp
119	(a) Although lookbehind assertions in PCRE must match fixed length strings,
120	each alternative branch of a lookbehind assertion can match a different length
121	of string. Perl requires them all to have the same length.
122	.sp
123	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
124	meta-character matches only at the very end of the string.
125	.sp
126	(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
127	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
128	(Perl can be made to issue a warning.)
129	.sp
130	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
131	inverted, that is, by default they are not greedy, but if followed by a
132	question mark they are.
133	.sp
134	(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
135	only at the first matching position in the subject string.
136	.sp
137	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
138	PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
139	.sp
140	(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
141	by the PCRE_BSR_ANYCRLF option.
142	.sp
143	(h) The callout facility is PCRE-specific.
144	.sp
145	(i) The partial matching facility is PCRE-specific.
146	.sp
147	(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
148	different hosts that have the other endianness.
149	.sp
150	(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
151	different way and is not Perl-compatible.
152	.sp
153	(l) PCRE recognizes some special sequences such as (*CR) at the start of
154	a pattern that set overall options that cannot be changed within the pattern.
155	.
156	.
157	.SH AUTHOR
158	.rs
159	.sp
160	.nf
161	Philip Hazel
162	University Computing Service
163	Cambridge CB2 3QH, England.
164	.fi
165	.
166	.
167	.SH REVISION
168	.rs
169	.sp
170	.nf
171	Last updated: 24 August 2011
172	Copyright (c) 1997-2011 University of Cambridge.
173	.fi
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