Exlude Real Directories From Mod_Rewrite

I'm looking for a way to exclude actual directories from mod_rewrite.

For example, at present my URL's look like this:
http://www.domain.com/category-name/

This it being rewritten to pass the category-name on to a PHP script.

However, if the user tries to go to a folder inside the main area, it's not allowed because right now everything is being considered 'rewritten' by the server if it's not just a file.

Here is what my .htaccess looks like right now.

RewriteEngine On
RewriteBase /
RewriteRule ^([a-zA-Z0-9\-]+)/?$ sites.php?cat=$1

Thoughts please?

 

RewriteEngine on
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([a-zA-Z0-9\-]+)/?$ sites.php?cat=$1

Code (markup):

The above only allows the rewrite rule to run if you are not referencing a file for folder that actually exists.