RL1.3 Subtraction and Intersection
To meet this requirement, an implementation shall supply
mechanisms for union, intersection and set-difference of
Unicode sets.
Java meets this requirement. However, because RL1.2 is not met,
it is of limited practical usefulness.
This is one of the things I really like about Java regular expressions.
It uses a different notation than given in the standard, but that's ok.
http://download.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html
[abc] a, b, or c (simple class)
[^abc] Any character except a, b, or c (negation)
[a-zA-Z] a through z or A through Z, inclusive (range)
[a-d[m-p]] a through d, or m through p: [a-dm-p] (union)
[a-z&&[def]] d, e, or f (intersection)
[a-z&&[^bc]] a through z, except for b and c: [ad-z] (subtraction)
[a-z&&[^m-p]] a through z, and not m through p: [a-lq-z](subtraction)
Although can *can* do all these using an appropriate combination
of lookahead assertions and negations, but doing so can be inconvient.
--tom
