This will not work if all tokens are not the same length, because of the reliance on index, which seems ok given the sample dataset provided. Also, its probably not the best approach, but I believe it works and I had fun producing it. 

use strict;
use warnings;

my $val = 'X';

my $min_len_subgroup;
sub chk {
  my $subgroup = shift;
  if(-1 != index($subgroup, $val)) {
    if (!defined $min_len_subgroup || length $subgroup < length $min_l
+en_subgroup) {
      $min_len_subgroup = $subgroup;
    }
  }
}

our $re = qr{
  (
   \(
     (?:
       (?> [^()]+ )
          |
       (??{ $re })
     )*
   \)
  )
  (?{ chk($1) })
}x;

my @pats = (
  "(1(2(X)))",
  "(1(2(X,a))) (3(X,e,f))",
  "(1(2(X,b,c))) (3(X,f))",
  "(1(2(X,a))) (X,b)",
  "(1(2(X,a))) (X,b) X",
  "(1(2(X,a))) (X,b) (X)",
);

for my $pat (@pats) {
  undef $min_len_subgroup;
  if (() = ($pat =~ /$re/g)) {
    print "$min_len_subgroup\n";
  }
}

[download]
,welchavw

In reply to Re: Parsing nested parentheses by welchavw
in thread Parsing nested parentheses by JeffR100

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