There is a regular-expression crossword puzzle floating around the internet, apparently from this year’s MIT Mystery Hunt: http://www.coinheist.com/rubik/a_regular_crossword/grid.pdf
Regular expression crossword puzzle
I believe that there is a solution at http://z.manicsages.org/puzzle/uploads/puzzle_files/50d697158c9cf.svg but I’ve not looked to see. The puzzle seems fairly difficult to me, but I’ve not spent much time on it.
