title: Regex to match 2-20 alphanumeric characters, allowing a single hyphen anywhere in the string
body: <p>I'm trying to learn how to use preg_match. I want users to be only allowed to sign up with username between 2-20 characters which can contain a-zA-Z0-9.</p>
  
  <p>Now the tricky part where Im getting lost, I want them to be able to include one hyphen anywhere in the username so,</p>
  
  <pre>
  -Brad = TRUE
  
  --Brad = FALSE
  
  B-Rad = TRUE
  </pre>