my $s = "abcxyQ";
    if ($s =~ /abc(?!xyz)/) { print "Matches '$s'\n"; }
    else { print "Doesn't Match '$s'\n"; }
    
    my $t = "abcxyz";
    if ($t =~ /abc(?!xyz)/) { print "Matches '$t'\n"; }
    else { print "Doesn't Match '$t'\n"; }