diff options
Diffstat (limited to 'pod/perlre.pod')
| -rw-r--r-- | pod/perlre.pod | 72 |
1 files changed, 57 insertions, 15 deletions
diff --git a/pod/perlre.pod b/pod/perlre.pod index 049894cc37..ae18614be4 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -1652,13 +1652,27 @@ matches a word that follows a tab, without including the tab in C<$&>. Prior to Perl 5.30, it worked only for fixed-width lookbehind, but starting in that release, it can handle variable lengths from 1 to 255 characters as an experimental feature. The feature is enabled -automatically if you use a variable length lookbehind assertion, but -will raise a warning at pattern compilation time, unless turned off, in -the C<experimental::vlb> category. This is to warn you that the exact -behavior is subject to change should feedback from actual use in the -field indicate to do so; or even complete removal if the problems found -are not practically surmountable. You can achieve close to pre-5.30 -behavior by fatalizing warnings in this category. +automatically if you use a variable length positive lookbehind assertion. + +In Perl 5.35.10 the scope of the experimental nature of this construct +has been reduced, and experimental warnings will only be produced when +the construct contains capturing parenthesis. The warnings will be +raised at pattern compilation time, unless turned off, in the +C<experimental::vlb> category. This is to warn you that the exact +contents of capturing buffers in a variable length positive lookbehind +is not well defined and is subject to change in a future release of perl. + +Currently if you use capture buffers inside of a positive variable length +lookbehind the result will be the longest and thus leftmost match possible. +This means that + + "aax" =~ /(?=x)(?<=(a|aa))/ + "aax" =~ /(?=x)(?<=(aa|a))/ + "aax" =~ /(?=x)(?<=(a{1,2}?)/ + "aax" =~ /(?=x)(?<=(a{1,2})/ + +will all result in C<$1> containing C<"aa">. It is possible in a future +release of perl we will change this behavior. There is a special form of this construct, called C<\K> (available since Perl 5.10.0), which causes the @@ -1712,16 +1726,44 @@ matches any occurrence of "foo" that does not follow "bar". Prior to Perl 5.30, it worked only for fixed-width lookbehind, but starting in that release, it can handle variable lengths from 1 to 255 characters as an experimental feature. The feature is enabled -automatically if you use a variable length lookbehind assertion, but -will raise a warning at pattern compilation time, unless turned off, in -the C<experimental::vlb> category. This is to warn you that the exact -behavior is subject to change should feedback from actual use in the -field indicate to do so; or even complete removal if the problems found -are not practically surmountable. You can achieve close to pre-5.30 -behavior by fatalizing warnings in this category. +automatically if you use a variable length negative lookbehind assertion. + +In Perl 5.35.10 the scope of the experimental nature of this construct +has been reduced, and experimental warnings will only be produced when +the construct contains capturing parentheses. The warnings will be +raised at pattern compilation time, unless turned off, in the +C<experimental::vlb> category. This is to warn you that the exact +contents of capturing buffers in a variable length negative lookbehind +is not well defined and is subject to change in a future release of perl. + +Currently if you use capture buffers inside of a negative variable length +lookbehind the result may not be what you expect, for instance: + + say "axfoo"=~/(?=foo)(?<!(a|ax)(?{ say $1 }))/ ? "y" : "n"; + +will output the following: + + a + no + +which does not make sense as this should print out "ax" as the "a" does +not line up at the correct place. Another example would be: + + say "yes: '$1-$2'" if "aayfoo"=~/(?=foo)(?<!(a|aa)(a|aa)x)/; + +will output the following: + + yes: 'aa-a' + +It is possible in a future release of perl we will change this behavior +so both of these examples produced more reasonable output. + +Note that we are confident that the construct will match and reject +patterns appropriately, the undefined behavior strictly relates to the +value of the capture buffer during or after matching. There is a technique that can be used to handle variable length -lookbehinds on earlier releases, and longer than 255 characters. It is +lookbehind on earlier releases, and longer than 255 characters. It is described in L<http://www.drregex.com/2019/02/variable-length-lookbehinds-actually.html>. |