I've never done anything significant with f?lex and it's been a long, long time since I've even read anything about its/their capabilities, but don't they have the same inherent "time-forward", if you will, orientation that regex compilers have?

IOW, if you had a flex process that parsed backwards, it would be easy to use, but isn't the big trick to design such a process in the first place? Can you expand on the capabilities of flex in this regard?