Possessive quantifiers are a way to prevent the regex engine from trying all permutations. This is primarily useful for performance reasons. You can also use possessive quantifiers to eliminate certain matches.
I've been thinking about the best approach to implement pure function verification in the Scala compiler. An approach similar to the one in D would fit a lot better than the one used in Haskell (which would break all existing code and cause some problems due to strict evaluation). A solution using annotations would be quite simple to implement: