Add documentation for obscure regex.

There are two issues here that need to be decided before merging:

1. Is it okay to include comments like this in podspecs (I think yes – @orta?)
2. Is this correct? (@tonyarnold, @ashfurrow)

I'm happy to revise this based on any feedback, though I do hope some documentation is included for the education of those who will come to maintain the pod.