Apache 2: Is Referer from current Host

I have an Apache 2 with mod_setenvif. My goal is to stop all hotlinking of images in my global apache.conf. Currently that's:

<FilesMatch ".(gif|jpg|jpeg|png)$">    SetEnvIfNoCase Referer "^http://[^/]*blogger.com/" hotlink    SetEnvIfNoCase Referer "^http://[^/]*myspace.com/" hotlink    SetEnvIfNoCase Referer "^http://[^/]*ebay" hotlink    ...    deny from env=hotlink</FilesMatch>

Works nicely so far, but I have to catch every hotlinker once and add it to my config. I would like to have a broader apprach by adding something like this:

    # Set variable "hotlink" if Referer contains "forum"    SetEnvIfNoCase Referer "forum" hotlink    # Unset variable if Referer is from the same Host as current request    SetEnvIfNoCase Referer %{Host} !hotlink

The plan is to match http://evilhost.com/forum/, but not http://myhost.com/forum/.

The problem is that the latter unset does not work. Looks as if I can't use the header property "Host" as regexp pattern - at least not the way I tried to. Of course I could manually enter all possible hostnames in my config, but that's exactly what I want to avoid.

So my question is:

• Is there a way to use a HTTP header as regexp pattern at all?
• If not, do you know another way I could reach my goal to unset the variable "hotlink" if the referrer is from the same host?