[Exim] ok rewrite question follow up but with regexpr ??

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
Ok following on from my rewrite mail
"[Exim] rewriting interesting query"

I have worked out a reg expressions to split the bits of my domain
user.somedomain.com up.

Obviously domains can be different and so i need somethign which just grabs
the first bit and then gives me whats left.

The reg expression is

$test = "something.somedomain.co.uk";
$test =~ m/(.+?)\.(.+)/;

print "front bit [$1]\n";
print "back bit [$2]\n";


Question is how to i get these 2 bits into exim???? How do you use a reg
expression in exim so i can perform this rewrite -

users_rewrite_doms:
driver = redirect
#domains = *.somedomain.co.uk"
domains = dbm;rewrite.db
#data = ${extract{1}{.}{$domain}}@???
data = < this is where i need reg expressino to grab bit 1 from $domain
then put an @ then get bit 2 from $domain


Please help

D

--