SharpC (LJ) is the author of this problem

In fact, the problem is a title of this topic :) Slight clarify — this can be solved using «canonical» regexp, it means concatenation + disjunction + Kleene star

P.S. well, in this problem the number is decimal, non-negative and any length

P.P.S. it's better to write regexps compatible with http://gskinner.com/RegExr/