Pass subdomain to rewrite rule. Nginx

nginx.conf

# Redirect http://test.domain.com to http://domain.com/?utm_source=test&utm_medium=423525

server {
    listen   80;
    server_name *.domain.com;
    if ($host ~* ^([a-z0-9-\.]+)\.domain\.com$) { set $subdomain $1; }
    rewrite ^ http://domain.com/?utm_source=$subdomain&utm_medium=$date_gmt permanent;
}