Klever

I expect that after #8425 we will have less problems with associating unknown marks with unknowns. Nevertheless, it seems that users often spend much time and create many useless versions of unknown marks when they try to specify correct regular expressions. Thus, it would be great to have an ability to see whether a given regular expression at least matches a given unknown. In ideal this check can be performed automatically in background after one types something as a regular expression, but also there can be a button for triggering this check. I am not sure that a check can be performed by JavaScript without requesting a server because of various libraries provide different regular expression engines and languages.