I can get lookahead to work with a single RegEx, but once I add anything between ( ) when using 'and', 'or' 'not' Booleans expressions JabRef doesn't recognize it in what it filters. ie:
(?=X) X, via zero-width positive lookahead
(?!X) X, via zero-width negative lookahead
(?<=X) X, via zero-width positive lookbehind
(?<!X) X, via zero-width negative lookbehind
This applies to ReGex Search and ReGex Groups Edit on JabRef Ver2.9.2
I have search everywhere, found no release notes, bugs, discussion or help file that clarifies the situation, and perhaps I didn't see where it said "JabRef does not handle this", in which case it may be a note required in the otherwise useful help.