Replace Header regex

Hi ,
I am trying to replace header using regex

The header is as
https://135.222.142.216:8443/cas/login?service=https://135.222.142.216:8443/search/login/cas

and i want to replace the first occurences of “8443” with another port for exanple 9999.

I tried with following, but they are not correct

http-response replace-header Location (.)(\b:8443/\b)(.)$ \1:"$port"/\3
http-response replace-header Location (.)(\b:8443/\b)(.)(?=?) \1:"$port"/\3

Any help would be much appreciated.
regards
Tridev