#!/usr/local/bin/perl
use strict;
use warnings;

#f = first, s = second.

my $f='ABCAAAABBBCCCCCAAABBBCCC';
my $s='ASRGRTGRT89579843rrrrrr';

print $f=~/(?=^[ABC]+$)/? "Matches [ABC] Combinations\n": "No pattern found!\n";
print $s=~/(?=^[ABC]+$)/? "Matches [ABC] Combinations\n": "No pattern found!\n";

##</code><code>##

#OUTPUT:
Matches [ABC] Combinations
No pattern found

##</code><code>##