Thus spoke Diez B. Roggisch (on 2006-10-08 10:49):
> Certainly true, and it always gives me a hard time because I don't know
> to which extend a regular expression nowadays might do the job because
> of these extensions. It was so much easier back in the old times....
Right, in perl, this would be a no-brainer,
its documented all over the place, like:
my $re;
$re = qr{
(?:
(?> [^\\()]+ | \\. )
|
\( (??{ $re }) \)
)*
}xs;
where you have a 'delayed execution'
of the
(??{ $re })
which in the end makes the whole a thing
recursive one, it gets expanded and
executed if the match finds its way
to it.
Above regex will match balanced parens,
as in:
my $good = 'a + (b / (c - 2)) * (d ^ (e+f)) ';
my $bad1 = 'a + (b / (c - 2) * (d ^ (e+f)) ';
my $bad2 = 'a + (b / (c - 2)) * (d) ^ (e+f) )';
if you do:
print "ok \n" if $good =~ /^$re$/;
print "ok \n" if $bad1 =~ /^$re$/;
print "ok \n" if $bad2 =~ /^$re$/;
This in some depth documented e.g. in
http://japhy.perlmonk.org/articles/tpj/2004-summer.html
(topic: Recursive Regexes)
Regards
M.
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