title: Rewrite URL by pre-pending a subdomain
body: <p>I am attempting to change a URL like the following:</p>
  
  <p>domain.com/123456  &lt;--- this can be any combination of any 6 digits</p>
  
  <p>to</p>
  
  <p>sub.domain.com/123456  &lt;----- this should be the same series of numbers as the original.</p>
  
  <p>Most tech notes I find are suggesting the opposite, that is, subdomain to domain, but I need domain to subdomain.</p>
  
  <p>Many thanks.</p>