mikejuk writes "Programmers often say that regular expressions are fun ... but now they can be a whole lot of fun in a completely new way. Want to try your hand at a regular expression crossword? 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 puzzle than crossword however because the clues determine the pattern that 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(pdf) was part of this year's MIT Mystery Hunt. 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."