[exim] DBM Lookup Question

Trying to do a DBM lookup.

condition =
${lookup{$domain}dbm{/etc/exim/control/run/alllocal.db}{yes}{no}}

But - if example.com is in the file it will also match xxx.example.com

How do I do a lookup where xxx.example.com won't match if example.com is
in the file but will match if xxx.example.com is in the file.

Thanks in advance.