#!/usr/bin/perl -w
use strict;

my @strings = ('ABCAAAABBBCCCCCAAABBBCCC',
               'ASRGRTGRT89579843rrrrrr',
               'A98797BqrtoiquyrtoCafdgagfd');                            
foreach (@strings)
{
   # Here the scalar count value of tr is used 
   # tr is very fast but lacks the flexibility of
   # regex
   # prints the string unless it has some char that is
   # not an A, B, C.
   
   print "$_\n" if !(tr/ABC//c);
}
# Prints: ABCAAAABBBCCCCCAAABBBCCC