Data group redirection or trim the hostname?

This should do it. Just create a class (type: string) with the name "class_exceptionList" and populate it with the folder names (in lower case) that shouldn't be redirected.

 when HTTP_REQUEST {

     extract the string between the first and second slash, 
     assuming that this is always the hostname to redirect to.
     Use a skip count of 1 to strip the first slash.
    set firstFolder [findstr [string tolower [HTTP::path]] "/" 1 "/"]

     Remove the first folder from the uri unless it's listed in the class.
     Using "string map" because it retains the case of the URI.
     (important for Linux/Apache, and also to retain the case 
     of any query strings)

    if { [class match $firstFolder equals class_exceptionList } {

        set uri [string map -nocase {"/$hostname" ""} [HTTP::uri]]

        HTTP::respond 301 Location "http://$firstFolder$uri"

    }

}