title: RegEx match character before digit but
body: <p>only after at least 3 characters and only one of those characters should be matched e.g. 
  for <code>lumia820</code> the match should be <code>a8</code> but for <code>aa6</code> there should not be any match.
  My current attempt is <code>/([a-z]{3,})([0-9])/</code>, however this wrongly includes the leading characters. This is probably an easy one for regex specialists but I am completely stuck here.. Can someone pls help?</p>