<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /dir/site_dir
RewriteCond %{REQUEST_URI} !^/admin
RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -dRewriteRule ^.*$ - [NC,L]
RewriteRule ^.*$ /dir/site_dir/index.php [NC,L]
</IfModule>