Khan/khan-exercises — pull request #46262 by petercollingridge (2013-05-29 19:55:19+00:00)
Title: Complex numbers captured by regex

Reattempting pull request #46233
For this Trello request: https://trello.com/card/adding-and-subtracting-complex-numbers-shouldn-t-accept-blank-answer/4d87e664967a0775082939ab/579