logging only when statment is true.

e.g.

when HTTP_REQUEST { 
  if { [string tolower [HTTP::path]] contains "/blah" } { 
    if { ! [class match [IP::client_addr] equals allowed_IPs]} { 
      log local0. "[IP::client_addr]:[TCP::client_port] does not match the allowed IP list"
      discard 
    } 
  } 
}