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revision 635 by ph10, Sat Jul 23 16:19:50 2011 UTC revision 716 by ph10, Tue Oct 4 16:38:05 2011 UTC
# Line 10  versions 5.10 and above. Line 10  versions 5.10 and above.
10  .P  .P
11  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what  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  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 assertions. Perl permits them, but  2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19  they do not mean what you might think. For example, (?!a){3} does not assert  not mean what you might think. For example, (?!a){3} does not assert that the
20  that the next three characters are not "a". It just asserts that the next  next three characters are not "a". It just asserts that the next character is
21  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 when followed by a character name or Unicode value. (\eN on its  \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  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  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  matching engine. If any of these are encountered by PCRE, an error is
# Line 53  the internal representation of Unicode c Line 50  the internal representation of Unicode c
50  implement the somewhat messy concept of surrogates."  implement the somewhat messy concept of surrogates."
51  .P  .P
52  7. PCRE implements a simpler version of \eX than Perl, which changed to make  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  \eX match what Unicode calls an "extended grapheme cluster". This is more
54  complicated than an extended Unicode sequence, which is what PCRE matches.  complicated than an extended Unicode sequence, which is what PCRE matches.
55  .P  .P
56  8. 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
# Line 82  the Line 79  the
79  .\"  .\"
80  documentation for details.  documentation for details.
81  .P  .P
82  10. Subpatterns that are called recursively or as "subroutines" are always  10. Subpatterns that are called as subroutines (whether or not recursively) are
83  treated as atomic groups in PCRE. This is like Python, but unlike Perl. There  always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
84  is a discussion of an example that explains this in more detail in the  There is a discussion of an example that explains this in more detail in the
85  .\" HTML <a href="pcrepattern.html#recursiondifference">  .\" HTML <a href="pcrepattern.html#recursiondifference">
86  .\" </a>  .\" </a>
87  section on recursion differences from Perl  section on recursion differences from Perl
# Line 95  in the Line 92  in the
92  .\"  .\"
93  page.  page.
94  .P  .P
95  11. There are some differences that are concerned with the settings of captured  11. If (*THEN) is present in a group that is called as a subroutine, its action
96    is limited to that group, even if the group does not contain any | characters.
97    .P
98    12. There are some differences that are concerned with the settings of captured
99  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
100  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".
101  .P  .P
102  12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern  13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
103  names is not as general as Perl's. This is a consequence of the fact the PCRE  names is not as general as Perl's. This is a consequence of the fact the PCRE
104  works internally just with numbers, using an external table to translate  works internally just with numbers, using an external table to translate
105  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
# Line 109  would not be possible to distinguish whi Line 109  would not be possible to distinguish whi
109  names map to capturing subpattern number 1. To avoid this confusing situation,  names map to capturing subpattern number 1. To avoid this confusing situation,
110  an error is given at compile time.  an error is given at compile time.
111  .P  .P
112  13. Perl recognizes comments in some places that PCRE does not, for example,  14. Perl recognizes comments in some places that PCRE does not, for example,
113  between the ( and ? at the start of a subpattern. If the /x modifier is set,  between the ( and ? at the start of a subpattern. If the /x modifier is set,
114  Perl allows whitespace between ( and ? but PCRE never does, even if the  Perl allows whitespace between ( and ? but PCRE never does, even if the
115  PCRE_EXTENDED option is set.  PCRE_EXTENDED option is set.
116  .P  .P
117  14. PCRE provides some extensions to the Perl regular expression facilities.  15. PCRE provides some extensions to the Perl regular expression facilities.
118  Perl 5.10 includes new features that are not in earlier versions of Perl, some  Perl 5.10 includes new features that are not in earlier versions of Perl, some
119  of which (such as named parentheses) have been in PCRE for some time. This list  of which (such as named parentheses) have been in PCRE for some time. This list
120  is with respect to Perl 5.10:  is with respect to Perl 5.10:
# Line 148  by the PCRE_BSR_ANYCRLF option. Line 148  by the PCRE_BSR_ANYCRLF option.
148  (i) The partial matching facility is PCRE-specific.  (i) The partial matching facility is PCRE-specific.
149  .sp  .sp
150  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on  (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
151  different hosts that have the other endianness.  different hosts that have the other endianness. However, this does not apply to
152    optimized data created by the just-in-time compiler.
153  .sp  .sp
154  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a  (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
155  different way and is not Perl-compatible.  different way and is not Perl-compatible.
# Line 171  Cambridge CB2 3QH, England. Line 172  Cambridge CB2 3QH, England.
172  .rs  .rs
173  .sp  .sp
174  .nf  .nf
175  Last updated: 23 July 2011  Last updated: 04 October 2011
176  Copyright (c) 1997-2011 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
177  .fi  .fi

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