[EXIM] Wildcards for virtual director

I've been trying to convert a legacy mail system to exim. It uses
sendmail's mailmaps driver. I've got everything working fine except for
one slight problem. In sendmail's mailmaps driver, you can have a domain
alias file that has addresses with subdomains in it, ie domain.com.map
has addresses with user@??? and user2@???. I can match
the one domain in the db file with dbm*:/file but the $domain variable
I'm using to key my lookup to the file for contains the subdomain, so
exim looks for a file called sub.domain.com.map instead of the file
domain.com.map. To make this clearer, here's the virtual driver.

virtual:
domains = dbm*;/opt/exim/etc/mailmaps,
driver = aliasfile,
no_more;
modemask = 002
file = /opt/exim/mailmaps/$domain.map,
search_type = lsearch*,
include_domain,
user = exim

Is there any way to strip the subdomain out of $domain before doing the
file lookup?

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