Much smaller regular expression to test divisibility by 7

I wonder if he built a state machine and then converted it to a regular expression.

He did you just need to look at the lhs to see how he does it with haskell.

Compatible with grep -Ex

See also http://github.com/matthiasgoergens/Div7 for the program that created it.

This is completely meaningless. The final regular expression is a complete black box and I dare people to try to decipher it.