irule to pick a number out of a URI and make it the port

It's not ideal but if you configure multiple Pools accordingly, with the Pool name ending in the port number it supports then the following pretty short rule should work as long as a four digit port number is always used. It is probably worth adding in some checks to ensure you have a valid value before rewriting the URI and Host and specifying the Pool.

when HTTP_REQUEST {
 Find the first / in the URI, skip 4 characters and match the rest of the string, set this as variable newuri
 set newuri [findstr [HTTP::uri] "/" "4"]
 Find the first / in the URI, skip 0 characters and match until the next /, this this as variable newport
 set newport [findstr [HTTP::uri] "/" "0" "/"]
 Change the URI before we send this request to a real server
 HTTP::uri "$newuri"
 Change the HTTP Host header
 HTTP::header replace Host "[HTTP::host]:$newport"
 Use the relevant Pool
 pool pool_$newport
}