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Sun Oct 31 14:06:43 2010 UTC (4 years, 5 months ago) by ph10
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Clarify documentation about comments in patterns.

1 nigel 79 .TH PCRECOMPAT 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
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 ph10 562 versions 5.10 and above.
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$xyz
66     .sp
67     The \eQ...\eE sequence is recognized both inside and outside character classes.
68     .P
69 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
70     constructions. However, there is support for recursive patterns. This is not
71 ph10 453 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
72 nigel 93 feature allows an external function to be called during pattern matching. See
73     the
74 nigel 75 .\" HREF
75     \fBpcrecallout\fP
76     .\"
77     documentation for details.
78     .P
79 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are always
80 ph10 461 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
81 ph10 453 is a discussion of an example that explains this in more detail in the
82     .\" HTML <a href="pcrepattern.html#recursiondifference">
83     .\" </a>
84     section on recursion differences from Perl
85     .\"
86     in the
87     .\" HREF
88 ph10 456 \fBpcrepattern\fP
89 ph10 453 .\"
90     page.
91 nigel 93 .P
92     10. There are some differences that are concerned with the settings of captured
93 nigel 63 strings when part of a pattern is repeated. For example, matching "aba" against
94     the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
95 nigel 75 .P
96 ph10 518 11. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
97 ph10 459 names is not as general as Perl's. This is a consequence of the fact the PCRE
98     works internally just with numbers, using an external table to translate
99     between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
100     where the two capturing parentheses have the same number but different names,
101     is not supported, and causes an error at compile time. If it were allowed, it
102     would not be possible to distinguish which parentheses matched, because both
103     names map to capturing subpattern number 1. To avoid this confusing situation,
104     an error is given at compile time.
105 ph10 456 .P
106 ph10 562 12. Perl recognizes comments in some places that PCRE doesn't, for example,
107     between the ( and ? at the start of a subpattern.
108     .P
109     13. PCRE provides some extensions to the Perl regular expression facilities.
110 ph10 456 Perl 5.10 includes new features that are not in earlier versions of Perl, some
111     of which (such as named parentheses) have been in PCRE for some time. This list
112     is with respect to Perl 5.10:
113     .sp
114     (a) Although lookbehind assertions in PCRE must match fixed length strings,
115     each alternative branch of a lookbehind assertion can match a different length
116     of string. Perl requires them all to have the same length.
117     .sp
118 nigel 63 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
119     meta-character matches only at the very end of the string.
120 nigel 75 .sp
121 nigel 63 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
122 ph10 181 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
123     (Perl can be made to issue a warning.)
124 nigel 75 .sp
125 nigel 63 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
126     inverted, that is, by default they are not greedy, but if followed by a
127     question mark they are.
128 nigel 75 .sp
129     (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
130     only at the first matching position in the subject string.
131     .sp
133     PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
134 nigel 75 .sp
135 ph10 231 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
136     by the PCRE_BSR_ANYCRLF option.
137 nigel 75 .sp
138 ph10 231 (h) The callout facility is PCRE-specific.
139 nigel 75 .sp
140 ph10 231 (i) The partial matching facility is PCRE-specific.
141     .sp
142     (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
143 nigel 75 different hosts that have the other endianness.
144 nigel 77 .sp
145 ph10 231 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
146 nigel 77 different way and is not Perl-compatible.
147 ph10 231 .sp
148     (l) PCRE recognizes some special sequences such as (*CR) at the start of
149     a pattern that set overall options that cannot be changed within the pattern.
150 ph10 99 .
151     .
152     .SH AUTHOR
153     .rs
154     .sp
155     .nf
156     Philip Hazel
157     University Computing Service
158     Cambridge CB2 3QH, England.
159     .fi
160     .
161     .
162     .SH REVISION
163     .rs
164     .sp
165     .nf
166 ph10 562 Last updated: 31 October 2010
167 ph10 518 Copyright (c) 1997-2010 University of Cambridge.
168 ph10 99 .fi


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