protocol Foo { init?(string: String) } class SubRegExp: NSRegularExpression { override init(pattern: String, options: NSRegularExpression.Options = []) throws { try super.init(pattern: pattern, options: options) } convenience init?(string: String) { try? self.init(pattern: string, options: .caseInsensitive) } required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") } } (edited)