Replace part of HTTP::host and permanently redirect

I'm looking at redirecting various hosts to a new domain.

Example 1: app.olddomain.com to app.newdomain.com Example 2: anotherapp.olddomain.com to anotherapp.newdomain.com

The problem I'm having (because I'm a newb with string manipulation) is switching that "olddomain" to the "newdomain".

I could do the following for each of the applications but it would mean a huge iRule:

when HTTP_REQUEST {
          if { [string tolower [HTTP::host]] equals "app.olddomain.com" } {
                    HTTP::respond 301 Location "https://app.newdomain.com[HTTP::uri]" 
          }
}

I was hoping to do something like this:

when HTTP_REQUEST {
          if {[HTTP::host] ends_with ".olddomain.com"} {
                    HTTP::respond 301 Location "https://--part of original host--.newdomain.com[HTTP::uri]"
           }
}

Is my idea sound? How do we accomplish the replace?

Thanks!