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#!/usr/bin/perl -w use strict; ... print "$_\n" if !(tr/ABC//c); } # Prints: ABCAAAABBBCCCCCAAABBBCCC
my $standard='ABCAAAABBBCCCCCAAABBBCCC'; ... #A98797BqrtoiquyrtoCafdgagfd #change to "not" of these: (!/[^$unique_letters]/); #to get: ABCAAAABBBCCCCCAAABBBCCC