how on earth do you even begin to think this twisted?

Trying to answer other people's questions is a very powerful technique for learning a subject more deeply yourself. In our normal lives, work (or play) tends to present us with a relatively static selection of problems to solve, and internal ("nope, too ugly") and external ("the in-house style guide") forces constrain our approaches to solving them. Dealing with someone else's problem, expressed in their own words and subject to their own constraints, can shake us from the shackles of habit upon our thoughts.

Another way to leap out of that rut is to create artificial constraints of our own. The disciplines of writing obfuscations or playing perl golf are examples of such constraints, but they are easy to create - yes, I know I could do that with a regexp in a loop, but can I do it with just a regexp and no loop? Or in one regexp instead of two? Ok, now I've done that - ugly though it is - can I think of input text that would break it? Learning stuff from books has its place, but I have always felt that something you've discovered for yourself is worth twice as much. So experiment.

I believe there is a very close relationship between the study of pattern (which is what regular expressions are all about) and the study of mathematics. A common mantra in mathematics is: so, you have this thing to prove, and you don't know how to prove it; so first, try proving something more specific - often that is easier, and maybe it'll give you a clue how to tackle the larger task. If that doesn't work (or even if it does), try proving something more general - paradoxically, sometimes that too turns out to be easier. I think BrowserUK's solution of matching more than you asked for is conceptually quite close to "proving something more general".

Hugo