irule: root domain (only) non-www to www redirect

I have a series of websites that need a non-www to www redirect, bu I still need to support direct subdomains without a redirect.

Example:

1. http://abc.com redirects to http://www.abc.com
2. http://bank.abc.com does not redirect and stays at http://bank.abc.com

Assume:

• I have a bunch of domains here and don't want write a conditional statement by domain, it's too brittle
• I have several subdomains to support and would prefer not to conditionally address each, but can if necessary
• I need to do http and https

A simple irule that handles 1, but not 2, http only:

    when HTTP_REQUEST {
  if { ! ([HTTP::host] starts_with "www.") } {
    HTTP::redirect http://www.[HTTP::host][HTTP::uri]
  }
}

What's an efficient way to say "redirect root domains only" ?