[Dovecot] Issue with converting users from cyrus user.domain.com

[Preston Lord]

I have been using cyrus for the past 6 years, but it is a pain to config. So
... I just installed dovecot 1.0.15 on a new debian 4r7 box.

In cyrus we were using usernames as "user.domain.tld".

In dovecot I realize I can:

auth_username_translation = .@

But this converts user.domain.tld to user at domain@tld which of course fails.
Is there a regex or something I can apply to pull this off?

I also tried 

auth_username_format = %n.%d

But I think this does the opposite of what I need.

Any help is GREATLY appreciated.

Thanks in advance

P.