Comment by eru

There's also redgrep (https://github.com/google/redgrep) that supports intersection and complements of regular expressions.

I am toying the idea of writing a little game where player A thinks of a regular expression, and player B tries to guess. If B guesses right, they win. If B guesses wrong, A has to provide a false positive and a false negative (if they exist), and B gets to guess again.

Can you think of ways to automate the roles of A and/or B?