title: Regex expression to match alpha character as first character and the rest alphanumeric
body: <p>Im trying to write a regex to match the following:</p>
  
  <pre><code>must start with a letter
  the rest must be alphanumeric 
  at least one character in length
  max characters 12
  </code></pre>
  
  <p>Should me able to match </p>
  
  <pre><code>a
  k2jdj
  hello
  example72812
  </code></pre>