前言：
以註冊登入來說，我們會希望使用者輸入帳號密碼的格式是正確的(英文、數字、電子郵件)
所以在使用 UITextField 時，可以加上正則表示式，來達成目的

正文：
class ViewController:後面加上 UITextFieldDelegat

class ViewController: UIViewController, UITextFieldDelegat {...}

我的做法是，先設定好格式

let regexPW = "^[A-Z][0-9A-Za-z]{3,9}$" //首字母大寫，加上首字至少4個字，最多10個，密碼格式
let regexEM = "^([a-z0-9_\\.-]+)@([\\da-z\\.-]+)\\.([a-z\\.]{2,6})$" //email格式

接著要設定 Textfield 的 delegate ，不然正則表示不會作用

override func viewDidLoad() {
        super.viewDidLoad()
        Textfield.delegate = self
        
    }

方法寫好後，可以做一個 Button 按下來觸發，這邊我是用 UILabel 配合 改顏色 來提醒格式錯誤

@IBAction func registerclick(_ sender: Any) {
    contentcheck()
}
func compare(regex: String,textField: UITextField) -> Int{
    let RE = try! NSRegularExpression(pattern: regex, options: .anchorsMatchLines)
    let matchs = RE.matches(in: textField.text!, options: .reportProgress, range: NSRange(location: 0, length: textField.text!.count))
    return matchs.count
        
    }
func contentcheck() -> Bool {
    if compare(regex: regexEM, textField: registeraccounttextfield) == 1 {
        registeraccounttextfield.backgroundColor = #colorLiteral(red: 0.9999960065, green: 1, blue: 1, alpha: 1)
    } else {
        registeraccounttextfield.backgroundColor = #colorLiteral(red: 0.9942680001, green: 0.8618952632, blue: 0.8470022082, alpha: 1)
        registeraccounttextfield.text = ""
        registeraccountlabel.text = "請輸入email格式！"
        return false
    }
    if compare(regex: regexPW, textField: registerpasswordtextfield) == 1 {
        registerpasswordtextfield.backgroundColor = #colorLiteral(red: 0.9999960065, green: 1, blue: 1, alpha: 1)
    }else {
        registerpasswordtextfield.backgroundColor = #colorLiteral(red: 0.9942680001, green: 0.8618952632, blue: 0.8470022082, alpha: 1)
        registerpasswordtextfield.text = ""
        registerpasswordlabel.text = "請輸入4~10個字元，第一個字為英文大寫"
        return false
    }
    if compare(regex: regexPW, textField: registerpasswordagaintextfield) == 1 {
        registerpasswordagaintextfield.backgroundColor = #colorLiteral(red: 0.9999960065, green: 1, blue: 1, alpha: 1)
    }else {
        registerpasswordagaintextfield.backgroundColor = #colorLiteral(red: 0.9942680001, green: 0.8618952632, blue: 0.8470022082, alpha: 1)
        registerpasswordagaintextfield.text = ""
        registerpasswordagainlabel.text = "請輸入正確的密碼！"
        return false
    }
    return true
}