[Exim] using regex in address list

I'm misunderstanding something about using regular expressions in an
address list lookup. I have this in my data acl:

warn    condition = ${lookup{$sender_address} \

lsearch{/opt/local/etc/eximconfig/maillist.address}{yes}{no}}
       set acl_m3 = Mailing List $sender_address

maillist.address is just a list of the envelope senders for the mail
lists to which I subscribe. But the exiscan list never has the same
envelope sender twice, so needs a regex to match. So my list looks like:

\N^2_return_924\..+@duncanthrax\.net\N
exim-users-admin@???
...

From the first bulleted item in section 10.16 of the exim spec docs, I
thought this was what was expected, but the condition fails to match
the regex entry.

Could someone clarify for me why this doesn't work?

Thanks,
Brad