@baskerville: Yep, I should have made that clearer. Would your example work, given the "sandwich", or would that need some trickery like @frostschutz did?
(I'm not completely terrible at regexps, but I'm no wizard either, so, err, small words, please ;p).
EDIT: If I put my brain in "regexp mode", I think it looks fine for that purpose, but I haven't tested it

. (i.e., accepts zero or more "classic" directories in front of an hidden one).
EDITē: The first group might fail to match for a single-character folder name, though, right?