Grab last octet of IP address and redirect

I want to redirect all request coming to my virtual server listening at 10.12.13.X to 10.50.10.X.

 

 

In version 9.x i put the following on all the 10.12.13.X virtual servers to redirect them to the correspoing 10.50.10.X virtual server on another load balancer.

 

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HTTP::redirect http://10.50.10.[getfield [IP::local_addr] . 4][HTTP::uri]

 

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How do i translate the above 9.x rule to a 4.x rule?