仅接受注册的字母字符（Swift）

Question

此前曾被标记为另一个问题的副本，但我问的是不同的。我问如何通过＆＃39; if＆＃39;来实现它。言。

我在我的应用中创建一个注册页面，我希望它只检查字母字符。我打算通过一个if声明。像这样：

var allowedCharArray = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

if usernameTextField.text.characters != (allowedCharArray Characters) && passwordTextField.text.characters != (allowedCharArray Characters) { 
//continue 
} else {
//popup error
}

我使用的语言很快。我如何设置这个＆＃39;如果＆＃39;声明？有更好的方法吗？