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1=head1 NAME
2
3perldelta - what is new for perl v5.9.5
4
5=head1 DESCRIPTION
6
7This document describes differences between the 5.9.4 and the 5.9.5
8development releases. See L<perl590delta>, L<perl591delta>,
9L<perl592delta>, L<perl593delta> and L<perl594delta> for the differences
10between 5.8.0 and 5.9.4.
11
12=head1 Incompatible Changes
13
14=head1 Core Enhancements
15
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16=head2 Regular expressions
17
18=over 4
19
20=item Recursive Patterns
21
22It is now possible to write recursive patterns without using the C<(??{})>
23construct. This new way is more efficient, and in many cases easier to
24read.
25
26Each capturing parenthesis can now be treated as an independent pattern
27that can be entered by using the C<(?PARNO)> syntax (C<PARNO> standing for
28"parenthesis number"). For example, the following pattern will match
29nested balanced angle brackets:
30
31 /
32 ^ # start of line
33 ( # start capture buffer 1
34 < # match an opening angle bracket
35 (?: # match one of:
36 (?> # don't backtrack over the inside of this group
37 [^<>]+ # one or more non angle brackets
38 ) # end non backtracking group
39 | # ... or ...
40 (?1) # recurse to bracket 1 and try it again
41 )* # 0 or more times.
42 > # match a closing angle bracket
43 ) # end capture buffer one
44 $ # end of line
45 /x
46
47Note, users experienced with PCRE will find that the Perl implementation
48of this feature differs from the PCRE one in that it is possible to
49backtrack into a recursed pattern, whereas in PCRE the recursion is
50atomic or "possessive" in nature.
51
52=item Named Capture Buffers
53
54It is now possible to name capturing parenthesis in a pattern and refer to
55the captured contents by name. The naming syntax is C<< (?<NAME>....) >>.
56It's possible to backreference to a named buffer with the C<< \k<NAME> >>
57syntax. In code, the new magical hash C<%+> can be used to access the
58contents of the buffers.
59
60Thus, to replace all doubled chars, one could write
61
62 s/(?<letter>.)\k<letter>/$+{letter}/g
63
64Only buffers with defined contents will be "visible" in the hash, so
65it's possible to do something like
66
67 foreach my $name (keys %+) {
68 print "content of buffer '$name' is $+{$name}\n";
69 }
70
71Users exposed to the .NET regex engine will find that the perl
72implementation differs in that the numerical ordering of the buffers
73is sequential, and not "unnamed first, then named". Thus in the pattern
74
75 /(A)(?<B>B)(C)(?<D>D)/
76
77$1 will be 'A', $2 will be 'B', $3 will be 'C' and $4 will be 'D' and not
78$1 is 'A', $2 is 'C' and $3 is 'B' and $4 is 'D' that a .NET programmer
79would expect. This is considered a feature. :-)
80
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81=item Possessive Quantifiers
82
83Perl now supports the "possessive quantifier" syntax of the "atomic match"
84pattern. Basically a possessive quantifier matches as much as it can and never
85gives any back. Thus it can be used to control backtracking. The syntax is
86similar to non-greedy matching, except instead of using a '?' as the modifier
87the '+' is used. Thus C<?+>, C<*+>, C<++>, C<{min,max}+> are now legal
88quantifiers.
89
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90=back
91
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92=head2 The C<_> prototype
93
94A new prototype character has been added. C<_> is equivalent to C<$> (it
95denotes a scalar), but defaults to C<$_> if the corresponding argument
96isn't supplied. Due to the optional nature of the argument, you can only
97use it at the end of a prototype, or before a semicolon.
98
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99=head1 Modules and Pragmas
100
101=head2 New Core Modules
102
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103=head2 Module changes
104
105=over 4
106
107=item C<base>
108
109The C<base> pragma now warns if a class tries to inherit from itself.
110
111=back
112
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113=head1 Utility Changes
114
115=head1 Documentation
116
117=head1 Performance Enhancements
118
119=head1 Installation and Configuration Improvements
120
121=head1 Selected Bug Fixes
122
123=head1 New or Changed Diagnostics
124
125=head1 Changed Internals
126
127=head1 Known Problems
128
129=head2 Platform Specific Problems
130
131=head1 Reporting Bugs
132
133If you find what you think is a bug, you might check the articles
134recently posted to the comp.lang.perl.misc newsgroup and the perl
135bug database at http://rt.perl.org/rt3/ . There may also be
136information at http://www.perl.org/ , the Perl Home Page.
137
138If you believe you have an unreported bug, please run the B<perlbug>
139program included with your release. Be sure to trim your bug down
140to a tiny but sufficient test case. Your bug report, along with the
141output of C<perl -V>, will be sent off to perlbug@perl.org to be
142analysed by the Perl porting team.
143
144=head1 SEE ALSO
145
146The F<Changes> file for exhaustive details on what changed.
147
148The F<INSTALL> file for how to build Perl.
149
150The F<README> file for general stuff.
151
152The F<Artistic> and F<Copying> files for copyright information.
153
154=cut