Matching nested parens

BernieC has asked for the wisdom of the Perl Monks concerning the following question:

Supersearch didn't turn anything up and I can't figure out if Parse::RecDescent will do this:

What I want is a subroutine that matches a set of nested parens. That is, what I'd like is:

$str = "(a a (a a)(a a))(b b(b)b b)"
and have
my ($parens, $rest) = match($str) ;
[download]

and have $parens end up with "a a (a a)(a a)" and $rest be "(b b(b)b b)" I seem to recall doing something like this when I was an undergraduate and it involved something like counting parens [that is, count +1 for a ( and -1 for a ) and you've got the first balanced set when the count == 0. Is there a more elegant way to do it in Perl?

Comment on Matching nested parens Download Code

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Re: Matching nested parens by haukex (Archbishop) on Oct 01, 2020 at 19:41 UTC
I would suggest Regexp::Common: `use warnings; use strict; use Data::Dump; use Regexp::Common qw/balanced/; "(a a (a a)(a a))(b b(b)b b)" =~ /$RE{balanced}{-parens=>'()'}/; dd $&, $'; # ("(a a (a a)(a a))", "(b b(b)b b)")` [download] Though of course there are plenty of other solutions, including Parse::RecDescent, though that might be a little overkill if this is all you're doing. See e.g. regex for nested "<"/">', and note that Text::Balanced is a core module.	[reply] [d/l]
Re^2: Matching nested parens by BernieC (Pilgrim) on Oct 01, 2020 at 21:24 UTC
Thanks for the advice. Looks like Regexp::Common is just what I wanted {I've just cpan installed it :o)}	[reply]
Re^3: Matching nested parens by BernieC (Pilgrim) on Oct 01, 2020 at 22:25 UTC
A footnote. I was curious about Regexp::Common. So: `use Regexp::Common qw/balanced/; say $RE{balanced}{-parens=>'()'} ;` [download] And I can now see why I didn't have a hope of figuring out the RE on my own... :o)	[reply] [d/l]
Re^4: Matching nested parens by AnomalousMonk (Archbishop) on Oct 01, 2020 at 23:24 UTC
Re^5: Matching nested parens by BernieC (Pilgrim) on Oct 06, 2020 at 15:29 UTC
Re: Matching nested parens by perlfan (Vicar) on Oct 06, 2020 at 16:50 UTC
An alternative approach is to iterate over each charactor; when you see a `(` push it on the stack array (and change some "state" variable, maybe a "depth count" which is also the size of the stack); then when you see a `)`, pop the top of the stack (will be a `(`). Based on the depth of the nesting (or size of the stack) you can do all sorts of things with the intervening characters you see.	[reply] [d/l] [select]
Re^2: Matching nested parens by jcb (Parson) on Oct 06, 2020 at 23:39 UTC
Note that there is no need to track stack depth independently in Perl if you use an array as your stack with the push and pop builtins: Perl will keep track of the array length, which is accessible either by using the array in scalar context or with the `$#` sigil. There is an important difference: `scalar @array` gives the number of elements in `@array` (`0` if `@array` is empty, `1` if there is one element) while `$#array` gives the highest index in `@array` (`-1` if `@array` is empty, `0` if there is one element).	[reply] [d/l] [select]


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