This seems like a good reason to simply translate extglobs into regexes, which should run in linear time, rather than put effort into building and debugging a parallel implementation.
-Martin PS: While we're about it, can we please fix the expansion of « a/*/b/c/d/* » so it only calls readdir on a/ and each dir matching a/*/b/c/d/ and NOT call readdir on dirs matching a/*/, a/*/b/, or a/*/b/c/.
