Can't see how this works either, because the regexp takes integers occuring within the string as characters and not as integers already representing the length of a character sequence. Correct me if I'm wrong but at least that's my interpretation of "Ac10y" as input having to be returned as "A11y" (10 + c = 10 + 1 = 11) within the given test, the solution presented here returns A3y by the way (how could it pass the test?).