 Character classes may appear within other character classes, and
    may be composed by the union operator (implicit) and the intersection
    operator (<tt>&amp;&amp;</tt>).
    The union operator denotes a class that contains every character that is
    in at least one of its operand classes.  The intersection operator
    denotes a class that contains every character that is in both of its
    operand classes.

    <p> The precedence of character-class operators is as follows, from
    highest to lowest