If you want Xapian search on a local maildir, I highly recommend notmuch[1]. Adding new mail and updating the index can take noticable time, but searching is super fast, it allows easy custom tagging, and search results are better than gmail in my experience.
I never used notmuch, but I was much inspired by it when I saw a talk about it at linux.conf.au a few years back. Later I ended up stealing several ideas from notmuch about how to interface with Xapian, like the one-letter prefixes.
Xapian supports a query syntax which uses a trailing '*', but it's not a true wildcard/regexp, it's a range search on an ordered index. Notmuch allows you to use that syntax, see http://notmuchmail.org/searching/
It can do simple wildcarding with the * character, but I'm not aware of any general regexp capability, same as with any indexed database. Since you still have the underlying maildirs you can always just use egrep I guess.
I use it from the emacs notmuch mode.
[1] http://notmuchmail.org/