I can't source this, but IIRC, Chrome devs have tried something like this, but couldn't get the match semantics in the RE2 case to exactly match existing Javasc

I can't source this, but IIRC, Chrome devs have tried something like this, but couldn't get the match semantics in the RE2 case to exactly match existing Javascript regex match semantics. I heard about this long ago from a friend, and I don't remember the details. But it's quite plausible. In general, "obvious" optimizations like this are almost always harder than they seem. One possible path would be to precisely characterize regexes on which both FSMs and backtrackers agree precisely, and of only those, use the FSM based approach. But still, even then, you're maintaining two regex engines instead of one, and both need to be production grade and very fast. It is no easy task.