title: QLineEdit Validation for {[A-Z] [a-z][0-9]} text input
body: <p>I want to accept a new username from the user in my application. I want the username string to contain only <code>A-Z</code> or <code>a-z</code> or <code>0-9</code>, and have a maxLength of 8. So I want to validate the input from <code>QLineEdit</code>. I went through the documentation but I am messed up with validators. How I can validate my <code>QLineEdit</code> for this purpose?</p>