That’s beautiful work. Check out other examples in the interactive web app:

https://ieviev.github.io/resharp-webapp/

Back in the Usenet days, questions came up all the time about matching substrings that do not contain whatever. It’s technically possible without an explicit NOT operator because regular languages are closed under complement — along with union, intersection, Kleene star, etc. — but a bear to get right by hand for even simple cases.

Unbounded lookarounds without performance penalty at search time are an exciting feature too.