#!/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)):