title: how to match specific combination of 2 letters with regex
body: <p>given this set of letters </p>
  
  <blockquote>
    <p>xx | af | an | bf | bn | cf | cn</p>
  </blockquote>
  
  <p>how can I see if, given two characters, they match against one of the above? </p>
  
  <p>I could easily hardcode the solution with a switch case, but I think regex is a more elegant solution. </p>