FAQ 4.23 How do I find matching/nesting anything?
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From: PerlFAQ Server on 10 Jul 2010 12:00 This is an excerpt from the latest version perlfaq4.pod, which comes with the standard Perl distribution. These postings aim to reduce the number of repeated questions as well as allow the community to review and update the answers. The latest version of the complete perlfaq is at http://faq.perl.org . -------------------------------------------------------------------- 4.23: How do I find matching/nesting anything? This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like "/x([^x]*)x/" will get the intervening bits in $1. For multiple ones, then something more like "/alpha(.*?)omega/" would be needed. But none of these deals with nested patterns. For balanced expressions using "(", "{", "[" or "<" as delimiters, use the CPAN module Regexp::Common, or see "(??{ code })" in perlre. For other cases, you'll have to write a parser. If you are serious about writing a parser, there are a number of modules or oddities that will make your life a lot easier. There are the CPAN modules "Parse::RecDescent", "Parse::Yapp", and "Text::Balanced"; and the "byacc" program. Starting from perl 5.8 the "Text::Balanced" is part of the standard distribution. One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time: while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { # do something with $1 } A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work: # $_ contains the string to parse # BEGIN and END are the opening and closing markers for the # nested text. @( = ('(',''); @) = (')',''); ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; @$ = (eval{/$re/},$@!~/unmatched/i); print join("\n",@$[0..$#$]) if( $$[-1] ); -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |