Character class subtraction
This will make it to match any single character present in one list (the character class), but not present in another list (the subtracted class). The syntax for this is [class-[subtract]]. If the character after a hyphen is an opening bracket, these flavors interpret the hyphen as the subtraction operator rather than the range operator.
Character class Intersection
You can intersect the same class more than once. [0-9&&[0-6&&[4-9]]] is the same as [4-6] as those are the only digits present in all three parts of the intersection. In Java and Ruby you can write the same regex as [0-9&&[0-6]&&[4-9]], [0-9&&[0-6&&4-9]], [0-9&&0-6&&[4-9]], or just [0-9&&0-6&&4-9].