Use regex to capture answer
Merge pull request #46373 from petercollingridge/multi-comp

Use regex to capture answer in multiply complex numbers to solve Trello issue: https://trello.com/card/exercises-shouldn-t-accept-blank-answers/4d87e664967a0775082939ab/579

Test plan: Try a few exercises and make sure the correct answer is still accepted

Auditors: eater, alpert