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revision 79 by nigel, Sat Feb 24 21:40:52 2007 UTC revision 678 by ph10, Sun Aug 28 15:23:03 2011 UTC
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
6  .sp  .sp
7  This document describes the differences in the ways that PCRE and Perl handle  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  regular expressions. The differences described here are with respect to Perl
9  5.8.  versions 5.10 and above.
10  .P  .P
11  1. PCRE does not have full UTF-8 support. Details of what it does have are  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12  given in the  it does have are given in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
13  .\" HREF  .\" HREF
14  \fBpcre\fP  \fBpcreunicode\fP
15  .\"  .\"
16  page.  page.
17  .P  .P
18  2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19  them, but they do not mean what you might think. For example, (?!a){3} does  not mean what you might think. For example, (?!a){3} does not assert that the
20  not assert that the next three characters are not "a". It just asserts that the  next three characters are not "a". It just asserts that the next character is
21  next character is not "a" three times.  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  .P
25  3. Capturing subpatterns that occur inside negative lookahead assertions are  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  counted, but their entries in the offsets vector are never set. Perl sets its
# Line 37  terminated by zero. The escape sequence Line 34  terminated by zero. The escape sequence
34  represent a binary zero.  represent a binary zero.
35  .P  .P
36  5. The following Perl escape sequences are not supported: \el, \eu, \eL,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37  \eU, and \eN. In fact these are implemented by Perl's general string-handling  \eU, and \eN when followed by a character name or Unicode value. (\eN on its
38  and are not part of its pattern matching engine. If any of these are  own, matching a non-newline character, is supported.) In fact these are
39  encountered by PCRE, an error is generated.  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  .P
43  6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is  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  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  tested with \ep and \eP are limited to the general category properties such as
46  Lu and Nd.  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  .P
56  7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in  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 $  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  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  variable interpolation (but of course PCRE does not have variables). Note the
# Line 62  following examples: Line 69  following examples:
69  .sp  .sp
70  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
71  .P  .P
72  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
73  constructions. However, there is support for recursive patterns using the  constructions. However, there is support for recursive patterns. This is not
74  non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
75  allows an external function to be called during pattern matching. See the  feature allows an external function to be called during pattern matching. See
76    the
77  .\" HREF  .\" HREF
78  \fBpcrecallout\fP  \fBpcrecallout\fP
79  .\"  .\"
80  documentation for details.  documentation for details.
81  .P  .P
82  9. There are some differences that are concerned with the settings of captured  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  strings when part of a pattern is repeated. For example, matching "aba" against
97  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
98  .P  .P
99  10. PCRE provides some extensions to the Perl regular expression facilities:  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
100  .sp  names is not as general as Perl's. This is a consequence of the fact the PCRE
101  (a) Although lookbehind assertions must match fixed length strings, each  works internally just with numbers, using an external table to translate
102  alternative branch of a lookbehind assertion can match a different length of  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
103  string. Perl requires them all to have the same length.  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  .sp
123  (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $  (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.  meta-character matches only at the very end of the string.
125  .sp  .sp
126  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
127  meaning is faulted.  meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
128    (Perl can be made to issue a warning.)
129  .sp  .sp
130  (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is  (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  inverted, that is, by default they are not greedy, but if followed by a
# Line 94  question mark they are. Line 134  question mark they are.
134  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried  (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.  only at the first matching position in the subject string.
136  .sp  .sp
137  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
138  options for \fBpcre_exec()\fP have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
 .sp  
 (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  
 matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  
 support.)  
 .sp  
 (h) PCRE supports named capturing substrings, using the Python syntax.  
139  .sp  .sp
140  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
141  package.  by the PCRE_BSR_ANYCRLF option.
142  .sp  .sp
143  (j) The (R) condition, for testing recursion, is a PCRE extension.  (h) The callout facility is PCRE-specific.
144  .sp  .sp
145  (k) The callout facility is PCRE-specific.  (i) The partial matching facility is PCRE-specific.
146  .sp  .sp
147  (l) The partial matching facility is PCRE-specific.  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
 .sp  
 (m) Patterns compiled by PCRE can be saved and re-used at a later time, even on  
148  different hosts that have the other endianness.  different hosts that have the other endianness.
149  .sp  .sp
150  (n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
151  different way and is not Perl-compatible.  different way and is not Perl-compatible.
152  .P  .sp
153  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
154  Last updated: 28 February 2005  a pattern that set overall options that cannot be changed within the pattern.
155  .br  .
156  Copyright (c) 1997-2005 University of Cambridge.  .
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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