Joseph Rezeau creates interesting "ad hoc" facility to catch missing words in his regexp question.
He seems to use "negative matching" for it implementation - i.e. it creates "negative" asnwer which works (displaying it's comments) when there are no match to regex defined in answer.
However, this feature needs serious discussion: it is quite unclear how it should work in general case, not "ad hoc" one with unclearly defined borders.
The main problems:
1. several different answers
2. grading issues, with mix of correct, incorrect and partially correct answers