[undefined procedure: HTTP:uri][HTTP:uri]

iRule Newbie question:

I figured this would be a straight forward attempt to reference a data group on a VIP using multiple hosts but apparently I was wrong, as I keep getting the subject error.

when HTTP_REQUEST { set pool_name [class match -value [string tolower [HTTP:uri]] starts_with datagroup_name] pool $pool_name }

The datagroup formatting is: hostname.name.domain:=poolname

I also tried the datagroup formatting as /hostname.name.domain:=poolname

I'm running 11.3. Appreciate your assistance.