Once you know how to write a regex to match a string with balanced parens, it's not to hard to write a regex to do what you want.

How does a string with balanced parens look like? It starts with an opening paren, then "junk" separated by strings with balanced parens, where "junk" is a string consisting of something that isn't a paren. Quite like a delimited string with escaped delimiters actually.

Now, how do we go from string with balanced parens to the requested string? Simple. We start with an opening paren, then junk string separated by strings with balanced parens, then "C", then junk strings separated by strings with balanced parens, and finally a closing paren.

So much for text. Here's the code:

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;
              
my $bal;
   $bal = qr /[(]
              [^()]*
           (?:(??{ $bal }) [^()]* )*
              [)]/x;
              
my $re  = qr /[(]
              [^()]*
           (?:(??{ $bal }) [^()]* )*
              C
              [^()]*
           (?:(??{ $bal }) [^()]* )*
              [)]/x;
    
    
while (<DATA>) {
    chomp;
    print "$_: ";
    print $& if /$re/;
    print "\n";
}


__DATA__
((A, B), C, (D, E))  
(((A, B), C, (D, E)))
((A, B), D, (C, E))
((A, C), D, (B, E))
(((A, C)), D, (B, E))
((A, (C)), D, (B, E))
A, B, C, D, E
((A, B), F, (D, E))



((A, B), C, (D, E)): ((A, B), C, (D, E))
(((A, B), C, (D, E))): ((A, B), C, (D, E))
((A, B), D, (C, E)): (C, E)
((A, C), D, (B, E)): (A, C)
(((A, C)), D, (B, E)): (A, C)
((A, (C)), D, (B, E)): (C)
A, B, C, D, E: 
((A, B), F, (D, E)):
[download]

(I cooked up this regex while I was walking home from the train station. It turned out to work at the first try).

Abigail

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.