One that seems to be missing from the list is Spencer’s, which I know of from being postgres’ (and tcl’s) and infamously supports backreferences while being quite resilient to the usual backtracking NFA issues.
It's not really "missing" because it's not intended to be an exhaustive list of all possible approaches. :-) It is merely a list of the regex engines currently in use in the regex crate.
It's essentially the same as RE2. But with the addition of a full DFA and a public versioned API for each of the internal engines. See: https://blog.burntsushi.net/regex-internals/
The regex crate follows the RE2 tradition of not supporting features that aren't known how to implement efficiently.
