title: c# Regex question: only letters, numbers and a dot (2 to 20 chars) allowed
body: <p>i am wrestling with my regex.
  I want to allow only letters and numbers and a dot in a username, and 2 to 20 chars long
  I thought of something like this</p>
  
  <p>[0-9a-zA-Z]{2,20}</p>
  
  <p>but then 21 chars is also ok, and that's not what i want</p>