Regular languages are closed under intersection (as well as difference, 
complement, and of course union).

I find myself needing to know whether two regexps intersect. That is, 
whether the intersection of the two regexps is not empty.

I could code it up using the algorithm which can be found just about 
anywhere regular language theory is sold, e.g. 
http://www.cs.may.ie/~jpower/Courses/parsing/node13.html

But I wonder if there's a better way to do this using ruby regular 
expression tricks, or at least a way that is already coded up for me?