Khan/khan-exercises — pull request #46233 by petercollingridge (2013-05-28 20:43:03+00:00)
Title: Use regex to capture complex number answer

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