Note that as a Mystery Hunt puzzle, the goal of the puzzle isn't just to fill in the grid, it's to find the answer, a secret word or phrase that would be filled into another puzzle (the metapuzzle).
The puzzles generally don't tell you how to extract the answer, but the idea is you know it when you see it.
I also created an HTML-based version of this one some time ago that allows rotation and color codes the rows as matching or not:
https://gregable.com/p/regexp-puzzle.html
