URL rewrite plus

I am stumbling with the same issue. I need to add "www" only if the person did not enter it as the host.

I am starting with the following iRule.

when HTTP_REQUEST {
if { [HTTP::uri] equals "/" } {
      HTTP::redirect https://[HTTP::host]/login.do
} else {
HTTP::redirect https://[HTTP::host][HTTP::uri]
} 
}

I am using the above rule as a generic redirect across 20 or so domain names. I only need to place the www in front if the HTTP::host only contains a root domain name.

For example I need the following to occur:

http://domain.com --> https://www.domain.com

http://host1.domain.com --> https://host1.domain.com

http://host5.site1.domain.com --> https://host5.site1.domain.com

Any help would be greatly appreciated.