title: preg_match regex required for specific UK postcode area code
body: <p>My regex knowledge isn't the best and although I've been looking through posts to get my head around it I'm unable to find what I'm looking for.</p>
  
  <p>I would like to use the following (where $postcode is the full postcode entered by the user):</p>
  
  <pre><code>if(preg_match("AREA CODE REGEX GOES HERE", $postcode))
  {
      //Do something   
  }
  </code></pre>
  
  <p>to do the following:</p>
  
  <p>Check where postcode area code entered matches one of the following DN15,16,17,18,19,20,21,22,31,32,33,34,35,36,37,38,39,40,41 (all DN).</p>
  
  <p>So I'm looking for the regex to match only those postcodes.  It could be DN21 1AB or DN21 2AB for example but it has to match the first part still of DN21.</p>
  
  <p>I hope that's enough information and thanks in advance for the help.</p>