use strict; use warnings; while (my $sequence = ) { chomp $sequence; if ($sequence =~ /[^ATCG]/){ warn "Sequence '$sequence' in line $. has invalid character after " . $-[0]; } else { print "Valid sequence: '$sequence'\n"; } } __DATA__ TAAGAACAATAAGAACAA TAAGAACAATAAUAACAA TAAGAACAATAAGAACAA