title: Add referrer code to url
body: <p>I need add the url of a wordpress blog a referrer code with this structure:</p>
  
  <p>http://domain/([a-zA-Z]{3}[0-9]{3}/...</p>
  
  <p>The site should work both with and without this code.
  If there is I have to use it, store it in session type.</p>
  
  <p>All through plugin so you do not have problems with upgrades or any wordpress plugin.</p>
  
  <p>I also tried to edit the file .htaccess with a custom RewriteRule but nothing.</p>
  
  <p>Ideas? Thanks!</p>
  
  <p>Sorry for bad english!</p>