search: parse regex holes in structural search

Created by: rvantonder

comby 0.18.* supports regular expressions inside hole syntax like :[hole~regex]. This regex should be extracted and used to filter files like we did before. But because regex can contain character class syntax like [...] inside holes, these templates need to be parsed now for balancedness, the previous regex to extract things can't recognize it.

I also refactored the functions and tests to be cleaner/clearer about the regex string that's generated.