[Exim] rewriting doesn't work?

why does the rule:

^([^!]+)!(.*)@local\.domain$ $2@$1 S

not work, I want the SMTP command: RCPT TO:<somewhere.com!
user@???> to be rewritten to <user@???> before actual
relay checking takes place, so it can be rejected because of relaying
rules after that... (this is what the manual says, is it not?)

I would like this to work because I have a default alias so now
<somewhere.com!user@???> gets deliverd in my default mailbox in
stead of rejected, even with the rewrite rule in place...
and when I test it with: exim -brw somewhere.com!user@???
I get the responce SMTP: user@???
and my exim is configured not to relay and thus it should reject
user@??? (it does when you try that directly)

B.T.W. I'm using Exim 3.22 on a 2.0.38 kernel (based on Slackware)

Bart