Rewrite subdomains to folders using nginx and dont change the address bar

Question

I want to forward requests getting to nginx like

example.com/blah

To my backend in this form:

blah.example.com/

The rewrite condition is just redirecting and I dont want the address bar to change.

if ($host ~* ^([^.]+\.[^.]+)$) {
  set $host_without_subdomain $1;
  rewrite ^\/(\w+)(\/.*)$ http://$1.$host_without_subdomain$2 break;
}

This is my conf:

upstream example {
    server unix:/data/example/shared/tmp/pids/example.sock;
}

server {
  listen 80;

  server_name *.example.com;

  root /data/example/current/public;

  access_log /data/nginx/log/example.access.log main;
  error_log /data/nginx/log/example.error.log notice;

  include /etc/nginx/common/app.conf;

  if ($host ~* ^([^.]+\.[^.]+)$) {
    set $host_without_subdomain $1;
    rewrite ^\/(\w+)(\/.*)$ http://$1.$host_without_subdomain$2 break;
  }

  location / {

    include /etc/nginx/common/proxy.conf;

    if (!-f $request_filename) {
      proxy_pass http://empowerkit;
      break;
    }
  }

  location = /500.html {
    root /data/example/current/public;
  }
}