htaccess for subdomains

How to translate the example URL to the needed one?

From

http://test.at.domain.com

To

http://domain.com/a开发者_C百科t/index.php?id=test

---

RewriteEngine on
RewriteCond %{HTTP_HOST} (\w+).at.domain.com [NC]
RewriteRule .* http://domain.com/at/index.php?id=%1 [L]