Rewrite/redirect subdomain URLs to subdirectory URLs

I'm restructuring an NGINX website from sub.domain.com to domain.com/sub and I'd like to dynamically rewrite/redirect all sub.domain.com/sub2 links to domain.com/sub/sub2.

At the moment, my rewrite rule just redirects sub.domain.com to domain.com/sub, but not sub.domain.com/sub2 to domain.com/sub/sub2, for example.

  server {
    server_name sub.domain.com;
    rewrite ^ $scheme://www.domain.com/sub$1 permanent;
  }