Based on http://www.coinheist.com/rubik/a_regular_crossword/grid.pdf.
Mouse over the grid to see the clues to this "crossword puzzle", and type letters into the boxes to solve it. The top set of boxes will turn white
when you have highlighted a box whose row satisfies the regex for that direction, and the letter boxes will turn grey when all three of their
constraints are satisfied.
Save your state: Click and then copy, then paste somewhere safe.