title: Check uppercase AND lowercase
body: <p>I'm searching from a while for the regular expression that matches a word containing between 8 and 20 characters combining uppercases AND lowercases:</p>
  
  <p>Here an example of valid expression : AaAaAaAaA or aaaaaaaaA</p>
  
  <p>Not Valid expression example : aaaaaaaaa or AAAAAAAA</p>
  
  <p>What I did till now is : ^[a-z][A-Z]{8,20}$, but as you can see, that doesn't work</p>
  
  <p>Somebody have an idea for that ?</p>
  
  <p>Best regards</p>