I built a similar library in TypeScript (also based on regex derivatives). You can really built cool tools with complement / intersection. E.g.

1) regex equivalence checker (check if intersection of complements is empty):

https://gruhn.github.io/regex-utils/equiv-checker.html

2) password generator from regex constraints (16+ chars, at least on upper case char, etc). Just take the intersection of all constraints and generate random matches from that:

https://gruhn.github.io/regex-utils/password-generator.html