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1 <html>
2 <head>
3 <title>pcrecompat specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrecompat man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <br><b>
17 </b><br>
18 <P>
19 This document describes the differences in the ways that PCRE and Perl handle
20 regular expressions. The differences described here are with respect to Perl
21 versions 5.10 and above.
22 </P>
23 <P>
24 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
25 it does have are given in the
26 <a href="pcreunicode.html"><b>pcreunicode</b></a>
27 page.
28 </P>
29 <P>
30 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31 not mean what you might think. For example, (?!a){3} does not assert that the
32 next three characters are not "a". It just asserts that the next character is
33 not "a" three times (in principle: PCRE optimizes this to run the assertion
34 just once). Perl allows repeat quantifiers on other assertions such as \b, but
35 these do not seem to have any use.
36 </P>
37 <P>
38 3. Capturing subpatterns that occur inside negative lookahead assertions are
39 counted, but their entries in the offsets vector are never set. Perl sets its
40 numerical variables from any such patterns that are matched before the
41 assertion fails to match something (thereby succeeding), but only if the
42 negative lookahead assertion contains just one branch.
43 </P>
44 <P>
45 4. Though binary zero characters are supported in the subject string, they are
46 not allowed in a pattern string because it is passed as a normal C string,
47 terminated by zero. The escape sequence \0 can be used in the pattern to
48 represent a binary zero.
49 </P>
50 <P>
51 5. The following Perl escape sequences are not supported: \l, \u, \L,
52 \U, and \N when followed by a character name or Unicode value. (\N on its
53 own, matching a non-newline character, is supported.) In fact these are
54 implemented by Perl's general string-handling and are not part of its pattern
55 matching engine. If any of these are encountered by PCRE, an error is
56 generated.
57 </P>
58 <P>
59 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
60 built with Unicode character property support. The properties that can be
61 tested with \p and \P are limited to the general category properties such as
62 Lu and Nd, script names such as Greek or Han, and the derived properties Any
63 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
64 Perl documentation says "Because Perl hides the need for the user to understand
65 the internal representation of Unicode characters, there is no need to
66 implement the somewhat messy concept of surrogates."
67 </P>
68 <P>
69 7. PCRE implements a simpler version of \X than Perl, which changed to make
70 \X match what Unicode calls an "extended grapheme cluster". This is more
71 complicated than an extended Unicode sequence, which is what PCRE matches.
72 </P>
73 <P>
74 8. PCRE does support the \Q...\E escape for quoting substrings. Characters in
75 between are treated as literals. This is slightly different from Perl in that $
76 and @ are also handled as literals inside the quotes. In Perl, they cause
77 variable interpolation (but of course PCRE does not have variables). Note the
78 following examples:
79 <pre>
80 Pattern PCRE matches Perl matches
82 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
83 \Qabc\$xyz\E abc\$xyz abc\$xyz
84 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
85 </pre>
86 The \Q...\E sequence is recognized both inside and outside character classes.
87 </P>
88 <P>
89 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
90 constructions. However, there is support for recursive patterns. This is not
91 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
92 feature allows an external function to be called during pattern matching. See
93 the
94 <a href="pcrecallout.html"><b>pcrecallout</b></a>
95 documentation for details.
96 </P>
97 <P>
98 10. Subpatterns that are called as subroutines (whether or not recursively) are
99 always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
100 Captured values that are set outside a subroutine call can be reference from
101 inside in PCRE, but not in Perl. There is a discussion that explains these
102 differences in more detail in the
103 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
104 in the
105 <a href="pcrepattern.html"><b>pcrepattern</b></a>
106 page.
107 </P>
108 <P>
109 11. If (*THEN) is present in a group that is called as a subroutine, its action
110 is limited to that group, even if the group does not contain any | characters.
111 </P>
112 <P>
113 12. There are some differences that are concerned with the settings of captured
114 strings when part of a pattern is repeated. For example, matching "aba" against
115 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
116 </P>
117 <P>
118 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
119 names is not as general as Perl's. This is a consequence of the fact the PCRE
120 works internally just with numbers, using an external table to translate
121 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
122 where the two capturing parentheses have the same number but different names,
123 is not supported, and causes an error at compile time. If it were allowed, it
124 would not be possible to distinguish which parentheses matched, because both
125 names map to capturing subpattern number 1. To avoid this confusing situation,
126 an error is given at compile time.
127 </P>
128 <P>
129 14. Perl recognizes comments in some places that PCRE does not, for example,
130 between the ( and ? at the start of a subpattern. If the /x modifier is set,
131 Perl allows whitespace between ( and ? but PCRE never does, even if the
132 PCRE_EXTENDED option is set.
133 </P>
134 <P>
135 15. PCRE provides some extensions to the Perl regular expression facilities.
136 Perl 5.10 includes new features that are not in earlier versions of Perl, some
137 of which (such as named parentheses) have been in PCRE for some time. This list
138 is with respect to Perl 5.10:
139 <br>
140 <br>
141 (a) Although lookbehind assertions in PCRE must match fixed length strings,
142 each alternative branch of a lookbehind assertion can match a different length
143 of string. Perl requires them all to have the same length.
144 <br>
145 <br>
146 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
147 meta-character matches only at the very end of the string.
148 <br>
149 <br>
150 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
151 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
152 (Perl can be made to issue a warning.)
153 <br>
154 <br>
155 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
156 inverted, that is, by default they are not greedy, but if followed by a
157 question mark they are.
158 <br>
159 <br>
160 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
161 only at the first matching position in the subject string.
162 <br>
163 <br>
165 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
166 <br>
167 <br>
168 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
169 by the PCRE_BSR_ANYCRLF option.
170 <br>
171 <br>
172 (h) The callout facility is PCRE-specific.
173 <br>
174 <br>
175 (i) The partial matching facility is PCRE-specific.
176 <br>
177 <br>
178 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
179 different hosts that have the other endianness. However, this does not apply to
180 optimized data created by the just-in-time compiler.
181 <br>
182 <br>
183 (k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
184 different way and is not Perl-compatible.
185 <br>
186 <br>
187 (l) PCRE recognizes some special sequences such as (*CR) at the start of
188 a pattern that set overall options that cannot be changed within the pattern.
189 </P>
190 <br><b>
192 </b><br>
193 <P>
194 Philip Hazel
195 <br>
196 University Computing Service
197 <br>
198 Cambridge CB2 3QH, England.
199 <br>
200 </P>
201 <br><b>
203 </b><br>
204 <P>
205 Last updated: 09 October 2011
206 <br>
207 Copyright &copy; 1997-2011 University of Cambridge.
208 <br>
209 <p>
210 Return to the <a href="index.html">PCRE index page</a>.
211 </p>


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