在textField輸入時，即時抓取text

//要用stringByReplacingCharactersInRange把textField delegate的回傳值給拚起來

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

        var username = (textField.text as NSString).stringByReplacingCharactersInRange(range, withString: string)

        if self.checkUsernameRule(username) {

            self.submitButton.enabled = true

        } else {

            self.submitButton.enabled = false

        }

        

        // return NO to not change text

        return true

    }

func checkUsernameRule(username: String) -> Bool {

        //Username should be 6–20 characters long, and start with a letter

        println(username)

        if username =~ "^[a-zA-Z]{1}(\\w{5,20})$" {

            return true

        }

        return false

    }