Reversing the string requires reversing the regex pattern too, and that's not trivial. For example, (?<=(.).*\1) would try to match \1.*(.), but \1 hasn't been populated yet.

Update:

... What am I saying! Just have people write (.).*\1. That's what they'd have to do with a right-to-left parser too. I suspect all issues can be solved with proper documentation.