EmailValidation.swift

func isValidEmailAddress(email: String) -> Bool {
        
        var returnValue = true
        let emailRegEx = "[A-Z0-9a-z.-_]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,3}"
        
        do {
            let regex = try NSRegularExpression(pattern: emailRegEx)
            let nsString = email as NSString
            let results = regex.matches(in: email, range: NSRange(location: 0, length: nsString.length))
            
            if results.count == 0
            {
                returnValue = false
            }
            
        } catch let error as NSError {
            print("invalid regex: \(error.localizedDescription)")
            returnValue = false
        }
        
        return  returnValue
   }

//Use with SkyFloatingLabelTextField

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        if self.emailTextField == textField{
            if let text = textField.text{
                let isEmailValid = isValidEmailAddress(emailAddressString: text)
                emailTextField.errorMessage = (isEmailValid) ? "" : "Invalid Email"
            }
        }
        return true
    }

//Use with default UITextfield and a Label to display error

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        if self.emailTextField == textField{
            if let text = textField.text{
                let isEmailValid = isValidEmailAddress(emailAddressString: text)
                errorLabel.text = (isEmailValid) ? "" : "Invalid Email"
                errorLabel.isHidden = isEmailValid
            }
        }
        return true
    }