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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 Unicode support. Details of what it does
25 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 sometimes
40 (but not always) sets its numerical variables from inside negative assertions.
41 </P>
42 <P>
43 4. Though binary zero characters are supported in the subject string, they are
44 not allowed in a pattern string because it is passed as a normal C string,
45 terminated by zero. The escape sequence \0 can be used in the pattern to
46 represent a binary zero.
47 </P>
48 <P>
49 5. The following Perl escape sequences are not supported: \l, \u, \L,
50 \U, and \N when followed by a character name or Unicode value. (\N on its
51 own, matching a non-newline character, is supported.) In fact these are
52 implemented by Perl's general string-handling and are not part of its pattern
53 matching engine. If any of these are encountered by PCRE, an error is
54 generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
55 \U and \u are interpreted as JavaScript interprets them.
56 </P>
57 <P>
58 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
59 built with Unicode character property support. The properties that can be
60 tested with \p and \P are limited to the general category properties such as
61 Lu and Nd, script names such as Greek or Han, and the derived properties Any
62 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
63 Perl documentation says "Because Perl hides the need for the user to understand
64 the internal representation of Unicode characters, there is no need to
65 implement the somewhat messy concept of surrogates."
66 </P>
67 <P>
68 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
69 between are treated as literals. This is slightly different from Perl in that $
70 and @ are also handled as literals inside the quotes. In Perl, they cause
71 variable interpolation (but of course PCRE does not have variables). Note the
72 following examples:
73 <pre>
74 Pattern PCRE matches Perl matches
76 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
77 \Qabc\$xyz\E abc\$xyz abc\$xyz
78 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
79 </pre>
80 The \Q...\E sequence is recognized both inside and outside character classes.
81 </P>
82 <P>
83 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
84 constructions. However, there is support for recursive patterns. This is not
85 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
86 feature allows an external function to be called during pattern matching. See
87 the
88 <a href="pcrecallout.html"><b>pcrecallout</b></a>
89 documentation for details.
90 </P>
91 <P>
92 9. Subpatterns that are called as subroutines (whether or not recursively) are
93 always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
94 Captured values that are set outside a subroutine call can be reference from
95 inside in PCRE, but not in Perl. There is a discussion that explains these
96 differences in more detail in the
97 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
98 in the
99 <a href="pcrepattern.html"><b>pcrepattern</b></a>
100 page.
101 </P>
102 <P>
103 10. If any of the backtracking control verbs are used in a subpattern that is
104 called as a subroutine (whether or not recursively), their effect is confined
105 to that subpattern; it does not extend to the surrounding pattern. This is not
106 always the case in Perl. In particular, if (*THEN) is present in a group that
107 is called as a subroutine, its action is limited to that group, even if the
108 group does not contain any | characters. Note that such subpatterns are
109 processed as anchored at the point where they are tested.
110 </P>
111 <P>
112 11. If a pattern contains more than one backtracking control verb, the first
113 one that is backtracked onto acts. For example, in the pattern
114 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
115 triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
116 same as PCRE, but there are examples where it differs.
117 </P>
118 <P>
119 12. Most backtracking verbs in assertions have their normal actions. They are
120 not confined to the assertion.
121 </P>
122 <P>
123 13. There are some differences that are concerned with the settings of captured
124 strings when part of a pattern is repeated. For example, matching "aba" against
125 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
126 </P>
127 <P>
128 14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
129 names is not as general as Perl's. This is a consequence of the fact the PCRE
130 works internally just with numbers, using an external table to translate
131 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b&#62;B),
132 where the two capturing parentheses have the same number but different names,
133 is not supported, and causes an error at compile time. If it were allowed, it
134 would not be possible to distinguish which parentheses matched, because both
135 names map to capturing subpattern number 1. To avoid this confusing situation,
136 an error is given at compile time.
137 </P>
138 <P>
139 15. Perl recognizes comments in some places that PCRE does not, for example,
140 between the ( and ? at the start of a subpattern. If the /x modifier is set,
141 Perl allows white space between ( and ? (though current Perls warn that this is
142 deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.
143 </P>
144 <P>
145 16. Perl, when in warning mode, gives warnings for character classes such as
146 [A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no
147 warning features, so it gives an error in these cases because they are almost
148 certainly user mistakes.
149 </P>
150 <P>
151 17. In PCRE, the upper/lower case character properties Lu and Ll are not
152 affected when case-independent matching is specified. For example, \p{Lu}
153 always matches an upper case letter. I think Perl has changed in this respect;
154 in the release at the time of writing (5.16), \p{Lu} and \p{Ll} match all
155 letters, regardless of case, when case independence is specified.
156 </P>
157 <P>
158 18. PCRE provides some extensions to the Perl regular expression facilities.
159 Perl 5.10 includes new features that are not in earlier versions of Perl, some
160 of which (such as named parentheses) have been in PCRE for some time. This list
161 is with respect to Perl 5.10:
162 <br>
163 <br>
164 (a) Although lookbehind assertions in PCRE must match fixed length strings,
165 each alternative branch of a lookbehind assertion can match a different length
166 of string. Perl requires them all to have the same length.
167 <br>
168 <br>
169 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
170 meta-character matches only at the very end of the string.
171 <br>
172 <br>
173 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
174 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
175 (Perl can be made to issue a warning.)
176 <br>
177 <br>
178 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
179 inverted, that is, by default they are not greedy, but if followed by a
180 question mark they are.
181 <br>
182 <br>
183 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
184 only at the first matching position in the subject string.
185 <br>
186 <br>
188 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
189 <br>
190 <br>
191 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
192 by the PCRE_BSR_ANYCRLF option.
193 <br>
194 <br>
195 (h) The callout facility is PCRE-specific.
196 <br>
197 <br>
198 (i) The partial matching facility is PCRE-specific.
199 <br>
200 <br>
201 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
202 different hosts that have the other endianness. However, this does not apply to
203 optimized data created by the just-in-time compiler.
204 <br>
205 <br>
206 (k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
207 <b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
208 and are not Perl-compatible.
209 <br>
210 <br>
211 (l) PCRE recognizes some special sequences such as (*CR) at the start of
212 a pattern that set overall options that cannot be changed within the pattern.
213 </P>
214 <br><b>
216 </b><br>
217 <P>
218 Philip Hazel
219 <br>
220 University Computing Service
221 <br>
222 Cambridge CB2 3QH, England.
223 <br>
224 </P>
225 <br><b>
227 </b><br>
228 <P>
229 Last updated: 10 November 2013
230 <br>
231 Copyright &copy; 1997-2013 University of Cambridge.
232 <br>
233 <p>
234 Return to the <a href="index.html">PCRE index page</a>.
235 </p>


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