abracad

I have a domain, eg dom*com, with various sub-domains, eg sub*dom*com. I would like to change the homepage of dom.com from index.html to index.php. To achieve this I added the line "redirect 301 /index.html http:--dom*com/index.php" to .htaccess. Although this works for the main domain it also redirects the homepages of the subdomains to dom*com/index.php. How can I restrict the re-direct to the root directory of the top domain and leave sub-domains and sub-directories untouched? (NB example domain names edited to allow posting)