Milter-greylist

I am seeing the following message in my log file:

Jun  3 08:39:47 john-desktop milter-greylist: unexpected void
backreference no 1 in regex /.*@\(.*\.\)*example\.com/ against
"<example@...>"

I recently updated to version 4.0 from 3.?.?.  I didn't have any
issues in 3.x with my list.

I think my regex looks right to me.  From looking at the code in acl.c
(around line 538 of the 4.0 code) I am guessing that it thinks that
every group must have a match, but I put things into a group because
they MAY match but don't have to, if that makes sense.

The purpose of the regex /.*@\(.*\.\)*example\.com/ is to whitelist a
sender if they are from the domain example.com.  So this includes
addresses like example@... along with
example@...

Is this a bug or am I doing something wrong?

Thanks,
John