Earlier today I read a blog post (via http://news.ycombinator.com/item?id=1937062 http://news.ycombinator.com/item?id=1937062), discussing the problem of findin

Earlier today I read a blog post (via http://news.ycombinator.com/item?id=1937062 http://news.ycombinator.com/item?id=1937062), discussing the problem of finding a regular expression that matches the binary numbers that are divisible by 3. The author of that post solved it more or less by trial-and-error, but – as some commenters on that thread already pointed out – there’s actually a fairly straightforward algorithm that works for any divisor in any base. I thought it would be a fun exercise to implement the algorithm in Javascript, and here is the result. It's quite fun to play with. Even though (obviously) I understand how it works, it's still somehow surprising to see a regular expression that matches all the multiples of 7 in base 10, for example. (That example is quite a hairy one!)