I'm a bit surprised that no one has suggested the standard greedy solution:

    s/(.*)b/$1X/
[download]

Depending on how many b's there are and where the last one is, that can be a lot more efficient than all those lookaheads. It also generalizes to patterns larger than a character, when the reverse solution would have to reverse the pattern as well, and the rindex wouldn't be able to pattern match at all. All in all, I'd say it's a pretty useful idiom to have in your toolbox.