title: How to check if string has letter in second character with regex
body: <p>Say I wanted to create an ID number such as <code>1A45</code> or <code>4F01</code>.</p>
  
  <p>What would the regex be to make sure that the string had exactly one letter as the second character?</p>
  
  <p>I am unsure how to check for specific combinations of characters.</p>
  
  <p>What I have so far is:</p>
  
  <pre><code>if(!Regex.IsMatch(txtTrainID.Text, @"^[\w,\d,\w,\w]+$"))
  </code></pre>
  
  <p>Which is obviously completely wrong, I've had trouble trying to find a decent simple answer to this anywhere.</p>