[squid-users] referer_regex and empty referer

I have a squid 2.5 cluster which is serving static images. I would like
to prevent servers from outside my domain from including these images in
their IMG tags. That part was fairly easy:

        acl images url_regex -i (gif|jpg)$
        acl example_referer referer_regex -i ^http://[^/]+\.example\.com
        http_access deny images !example_referer

However, I would like people to be able to load the images directly -
that is, I'd like the servers to allow requests that don't have a
Referer: header.

I tried this acl:

        acl no_referer referer_regex ^$

but it doesn't seem to match requests without a Referer.

Does anyone know a way to do this?

-- 
Kevin Murphy <kevin@eorbit.net>
"If you had a neck and I had hands, I would squeeze your brain, which is
your body, right out the top of your head, which does not exist!" - Carl