Thanks! All good clarifications. You're definitely right on all counts, and it was more than a bit naive of me to suggest that language/library authors "simply" have two production grade regex implementations :)

>> (And if you are using them, then I'd argue you're shoving a bit too much logic into a regex, but that's beside the point).

> This kind of ignores the fact that regexes are often the interface to something else.

I will definitely admit to using the occasional assertion or backreference in grep or sed from time to time, so you make a good point about local tools too. I suppose I was thinking more in terms of programming APIs when I said this, and so keeping the standard library FSM-based would still apply here. I'd imagine that special-purpose text munging tools which care that much, will likely have their own engines anyway.

Thanks also for your work on the regex crate. I'm still dipping my toes into Rust but I'm glad to know there's an FSM-based default regex library!