Auto-correction does not work

[hgeorgsch]

We need documentation of how to write the regexp to fit with MathQuill output. RegExp gives a nasty-looking feedback with strike-through etc., even if the answer is apparently correct. There could be trouble with escape characters, or something else.

It seems that RegExp is not an ideal parent class, and it may be better to change MathExp for an alternative ShortMath building on ShortAnswer.

[hgeorgsch]

We should create an alternative version based on Short Answer.
The use of regexps in RegExp is only partial, leaving no real benefit.