Destination based redirect irule, a'la cache.pac implementing?

  
  when HTTP_REQUEST {  
    
      Check if host header value only contains numerals or colons ":"  
      The format is {%[CHARS]}, where CHARS can be a character ranges or single characters.    
      The literal hyphern character '-' needs to be listed first or last in the character set  
     if {[scan [HTTP::host] {%[0-9:]}] eq [HTTP::host]}{  
    
         Split the host header value on the : if it exists   
           and check if it's part of the stock private_net datagroup  
        if {[matchclass [getfield [HTTP::host] ":" 1] eq $::private_net]}{  
    
            Use the direct pool  
           pool direct_pool  
             
         Stop processing this event in this rule  
        return  
        }  
     }  
      If we reached here, use the proxy pool  
     pool proxy_pool  
  }