my $x = "the quick brown fox";
if ($x =~ /[ayz]/) {print "first found\n"} #will fail since the string 
    # does NOT contain an a, y or z

if ($x =~ /[wxyz]/) {print "2nd found\n"} #will match since both w 
    # and x are present, one would be enough
    # [w-z] would have been equivalent and one keystroke less