I recently finished the Hamlet series of the Regex Crossword puzzles. These are crossword puzzles where clues are not in the format of sentences, but instead are regular expressions that the appropriate row or column must satisfy.  The website contains several collections of puzzles, each with a common theme such as cities or palindromes for example.  I enjoyed completing these puzzles and I’d recommend them to any regex fans.

For a more extreme challenge, check out the hexagonal regex crossword created for the 2013 MIT Mystery Hunt.