extension String {
    var isAlphanumeric: Bool {
        return !isEmpty && range(of: "[^a-zA-Z0-9]", options: .regularExpression) == nil
    }
}

"".isAlphanumeric        // false
"abc123".isAlphanumeric  // true
"iOS 9".isAlphanumeric   // false

if yourString.rangeOfCharacter(from: CharacterSet.alphanumerics.inverted) != nil {
    return "Username can only contain numbers or digits"
}

extension String {
    func isAlphanumeric() -> Bool {
        return self.rangeOfCharacter(from: CharacterSet.alphanumerics.inverted) == nil
    }
}

"Hello".isAlphanumeric()             == true
"Hello In English".isAlphanumeric()  == false
"HelloInEnglish".isAlphanumeric()    == true
"BonjourEnFrançais".isAlphanumeric() == true