htaccess Elite

Hi,
In order to standardized on a single use of either the / or /index.{ext} for my Joomla installation I added to my .htaccess:

RewriteEngine On
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /.*index\.php\ HTTP/
RewriteRule ^(.*)index\.php /$1 [R=301,L]

It works fine in the front-end but causes problems in the back-end (admin area):

http://www.domian.com/administrator.index.php

is being redirected to http://www.domian.com/administrator

how can I remove the administrator directory from the rewrite rule?