title: Regex for 2 letters followed by 4 digits
body: <p>The string should begin with "S2" followed by any 4 digits.</p>
  
  <p>A matching string is "S20165".</p>
  
  <p>I'm trying with the following code, but it always echos OK even when there are 5 or 6 digits.</p>
  
  <pre><code>$string='S20104';
  if(preg_match('/S2[0-9]{4}/', $string)){
      echo 'OK';
  }
  else{
      echo 'NOT OK';
  }
  </code></pre>