acl and multiple header values

[John Fieber]

I want to create an ACL based on X-Forwarded-For:

    acl whitelist hdr_ip(X-Forwarded-For) -f whitelist.txt
    block unless whitelist

Which is just grand, EXCEPT I'm only interested in (and trust) the last address 
in the X-Forwarded-For header.  The above acl matches any address in the 
header.  I've been digging for a good chunk of the day how to do that and come 
up empty handed.  Help?

Thanks,

-john