[eru]

For your entertainment, https://github.com/matthiasgoergens/Div7 has a regex finder for divisibility. E.g. https://raw.github.com/matthiasgoergens/Div7/master/regex7 checks decimal numbers for divisibility by 7.

[abecedarius]

Very cute. :-)

I found my code and it wasn't exactly it (it just enumerated all regexes in order of size), but here's a crude solution now: https://github.com/darius/sketchbook/blob/master/regex/find_...

(In defense of my memory, I had written superoptimizers for other things.)