findstr

I trim at the ! so I only get the session ID. If you trim at the ? you also get the unique weblogic server identifiers, which I have noticed occasionally change.

If you want to discover these unique identifiers real time, you can insert the header X-Weblogic-Request-ClusterInfo towards the application:

when CLIENT_ACCEPTED {
  set get_server_mappings 1
}
when HTTP_REQUEST {
  if { $get_server_mappings } {
    HTTP::header insert "X-Weblogic-Request-ClusterInfo" "true"
    set get_server_mappings 0
  }
}

Then on the next response, extract the information from the header X-WebLogic-Cluster-List

when HTTP_RESPONSE {
  if { [HTTP::header exists "X-WebLogic-Cluster-List"] }{
    set server_mappings [split [HTTP::header "X-WebLogic-Cluster-List"] "|" ]
     Remove header before returning to client
    HTTP::header remove "X-WebLogic-Cluster-List"
  }
}

I'll leave it to you to discover what to do with this information, weblogic returns a lot of useful information!