[Exim] domainlist question

I know how to make a domain list where the list is just a list of domains:

domainlist preprocess_domains = lsearch;/etc/exim/preprocess

BUT ----

Suppse the file has a domain list where the first field is the domain
and the second field is something else:

domain1.com:yes
domain2.com:yes
domain3.com:yes

I need a domain list of the first field. How do I do it?