Apache Reverse Proxy migration to F5.

I'm not quite sure what your question is - are you asking for help understanding what your current RewriteRule does (it seems to rewrite "/" to "/ptg/rm"), or are you asking how to implement the same rule in an iRule (or ltm policy), or is the question something else ?

If you were to use a policy, it would be something like this:

ltm policy rewriteurls {
    requires { http }
    rules {
        rewrite_root {
            actions {
                0 {
                    http-uri
                    replace
                    path /ptg/rm
                }
            }
            conditions {
                0 {
                    http-uri
                    path
                    values { / }
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}

Does that help ?