From: Robert Klemme <shortcutter@...>
Date: 2011-02-01T17:38:27+09:00
Subject: Re: Recursive regular expressions in Ruby?

On Tue, Feb 1, 2011 at 12:05 AM, Jarmo Pertman <jarmo.p@gmail.com> wrote:
> I was just wondering if there is possible to write recursive regular
> expressions in Ruby? As i'm understanding at this moment then there
> isn't. Prove me wrong.
>
> If you're not aware of such possibility in Perl for example then here
> is one good blog post about it http://www.catonmat.net/blog/recursive-regular-expressions
>
> It seems that also PHP has such a functionality.

Strictly speaking these are no more regular expressions any more
because the set of languages that they can detect exceeds the set of
regular languages.  Normally you need at least a parser for a context
free language to detect nested brackets etc.  (There are quite a few
parser generators available for Ruby.)

But: Oniguruma can do it, too:

Ruby version 1.9.2
irb(main):001:0> re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/
=> /((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/
irb(main):002:0> s = 'some(stri\)\((()x)(((c)d)e)\))ng'
=> "some(stri\\)\\((()x)(((c)d)e)\\))ng"
irb(main):003:0> mt = s.match re
=> #<MatchData "(stri\\)\\((()x)(((c)d)e)\\))"
pg:"(stri\\)\\((()x)(((c)d)e)\\))">
irb(main):004:0> mt[1]
=> "(stri\\)\\((()x)(((c)d)e)\\))"

http://www.siaris.net/index.cgi/+Programming/LanguageBits/Ruby/Oniguruma.rdoc

Kind regards

robert

-- 
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