The idea is simple enough — create a crossword style puzzle with regular expressions are the "clues". In case you don't know what a regular expression is — it is a way of specifying what characters are allowed using wild-card characters and more. For example a dot matches any single character, an * any number of characters and so on.
The regular expression crossword is more a sort of Sudoku style puzzle than crossword however because the clues determine the pattern the the entries in a row have to satisfy. It also has to use a hexagonal grid to provide three regular expressions to control each entry.
This particular regular expression crossword was part of this year's MIT Mystery Hunt, and if you don't know anything about it then good — because it could waste a lot of time. This annual event is crammed with a collection of very difficult problems and the regular expression crossword, created by Dan Gulotta from an idea by Palmer Mebane, was just a small part of the whole — and yes there is a solution.