=encoding utf8 =head1 NAME perl5100delta - what is new for perl 5.10.0 =head1 DESCRIPTION This document describes the differences between the 5.8.8 release and the 5.10.0 release. Many of the bug fixes in 5.10.0 were already seen in the 5.8.X maintenance releases; they are not duplicated here and are documented in the set of man pages named perl58[1-8]?delta. =head1 Core Enhancements =head2 The C pragma The C pragma is used to enable new syntax that would break Perl's backwards-compatibility with older releases of the language. It's a lexical pragma, like C or C. Currently the following new features are available: C (adds a switch statement), C (adds a C built-in function), and C (adds a C keyword for declaring "static" variables). Those features are described in their own sections of this document. The C pragma is also implicitly loaded when you require a minimal perl version (with the C construct) greater than, or equal to, 5.9.5. See L for details. =head2 New B<-E> command-line switch B<-E> is equivalent to B<-e>, but it implicitly enables all optional features (like C). =head2 Defined-or operator A new operator C (defined-or) has been implemented. The following expression: $a // $b is merely equivalent to defined $a ? $a : $b and the statement $c //= $d; can now be used instead of $c = $d unless defined $c; The C operator has the same precedence and associativity as C<||>. Special care has been taken to ensure that this operator Do What You Mean while not breaking old code, but some edge cases involving the empty regular expression may now parse differently. See L for details. =head2 Switch and Smart Match operator Perl 5 now has a switch statement. It's available when C is in effect. This feature introduces three new keywords, C, C, and C: given ($foo) { when (/^abc/) { $abc = 1; } when (/^def/) { $def = 1; } when (/^xyz/) { $xyz = 1; } default { $nothing = 1; } } A more complete description of how Perl matches the switch variable against the C conditions is given in L. This kind of match is called I, and it's also possible to use it outside of switch statements, via the new C<~~> operator. See L. This feature was contributed by Robin Houston. =head2 Regular expressions =over 4 =item Recursive Patterns It is now possible to write recursive patterns without using the C<(??{})> construct. This new way is more efficient, and in many cases easier to read. Each capturing parenthesis can now be treated as an independent pattern that can be entered by using the C<(?PARNO)> syntax (C standing for "parenthesis number"). For example, the following pattern will match nested balanced angle brackets: / ^ # start of line ( # start capture buffer 1 < # match an opening angle bracket (?: # match one of: (?> # don't backtrack over the inside of this group [^<>]+ # one or more non angle brackets ) # end non backtracking group | # ... or ... (?1) # recurse to bracket 1 and try it again )* # 0 or more times. > # match a closing angle bracket ) # end capture buffer one $ # end of line /x PCRE users should note that Perl's recursive regex feature allows backtracking into a recursed pattern, whereas in PCRE the recursion is atomic or "possessive" in nature. As in the example above, you can add (?>) to control this selectively. (Yves Orton) =item Named Capture Buffers It is now possible to name capturing parenthesis in a pattern and refer to the captured contents by name. The naming syntax is C<< (?....) >>. It's possible to backreference to a named buffer with the C<< \k >> syntax. In code, the new magical hashes C<%+> and C<%-> can be used to access the contents of the capture buffers. Thus, to replace all doubled chars with a single copy, one could write s/(?.)\k/$+{letter}/g Only buffers with defined contents will be "visible" in the C<%+> hash, so it's possible to do something like foreach my $name (keys %+) { print "content of buffer '$name' is $+{$name}\n"; } The C<%-> hash is a bit more complete, since it will contain array refs holding values from all capture buffers similarly named, if there should be many of them. C<%+> and C<%-> are implemented as tied hashes through the new module C. Users exposed to the .NET regex engine will find that the perl implementation differs in that the numerical ordering of the buffers is sequential, and not "unnamed first, then named". Thus in the pattern /(A)(?B)(C)(?D)/ $1 will be 'A', $2 will be 'B', $3 will be 'C' and $4 will be 'D' and not $1 is 'A', $2 is 'C' and $3 is 'B' and $4 is 'D' that a .NET programmer would expect. This is considered a feature. :-) (Yves Orton) =item Possessive Quantifiers Perl now supports the "possessive quantifier" syntax of the "atomic match" pattern. Basically a possessive quantifier matches as much as it can and never gives any back. Thus it can be used to control backtracking. The syntax is similar to non-greedy matching, except instead of using a '?' as the modifier the '+' is used. Thus C, C<*+>, C<++>, C<{min,max}+> are now legal quantifiers. (Yves Orton) =item Backtracking control verbs The regex engine now supports a number of special-purpose backtrack control verbs: (*THEN), (*PRUNE), (*MARK), (*SKIP), (*COMMIT), (*FAIL) and (*ACCEPT). See L for their descriptions. (Yves Orton) =item Relative backreferences A new syntax C<\g{N}> or C<\gN> where "N" is a decimal integer allows a safer form of back-reference notation as well as allowing relative backreferences. This should make it easier to generate and embed patterns that contain backreferences. See L. (Yves Orton) =item C<\K> escape The functionality of Jeff Pinyan's module Regexp::Keep has been added to the core. In regular expressions you can now use the special escape C<\K> as a way to do something like floating length positive lookbehind. It is also useful in substitutions like: s/(foo)bar/$1/g that can now be converted to s/foo\Kbar//g which is much more efficient. (Yves Orton) =item Vertical and horizontal whitespace, and linebreak Regular expressions now recognize the C<\v> and C<\h> escapes that match vertical and horizontal whitespace, respectively. C<\V> and C<\H> logically match their complements. C<\R> matches a generic linebreak, that is, vertical whitespace, plus the multi-character sequence C<"\x0D\x0A">. =back =head2 C say() is a new built-in, only available when C is in effect, that is similar to print(), but that implicitly appends a newline to the printed string. See L. (Robin Houston) =head2 Lexical C<$_> The default variable C<$_> can now be lexicalized, by declaring it like any other lexical variable, with a simple my $_; The operations that default on C<$_> will use the lexically-scoped version of C<$_> when it exists, instead of the global C<$_>. In a C