fancy-regex is really cool. It implements backtracking and look-around for regular expressions while delegating as much as possible to the regex crate.
In essence, it is a backtracking VM (as well explained in Regular Expression Matching: the Virtual Machine Approach) in which one of the "instructions" in the VM delegates to an inner NFA implementation (in Rust, the regex crate, though a similar approach would certainly be possible using RE2 or the Go regexp package). Then there's an analysis which decides for each subexpression whether it is "hard", or can be delegated to the NFA matcher. At the moment, it is eager, and delegates as much as possible to the NFA engine.