How to get domain name in iRule?

Certainly. Firstly, I assume you want to make sure that you match "foo" only when it is the complete name leaf, and outside of that, you will substitute any domain name which contains that leaf:

when HTTP_REQUEST {
    if { [HTTP::host] starts_with "foo." } {
        HTTP::redirect "https://bar.[string range [HTTP::host] 4 end][HTTP::uri]
    }
}

The

4

in the

string range

command is the length of the leaf name and its following dot (

foo.

, in this example). Notice, as well, that the leading slash (

/

) is not needed before

HTTP::uri

(although, strictly speaking, a user-agent could pass the protocol and/or host part in the Request-URI, but that's an entirely different problem).