Filter non-digits from string

Question

Using only swift code I cant figure out how to take \"(555) 555-5555\" and return only the numeric values and get \"5555555555\". I need to remove all the parentheses, whit

Answer 1

Swift 3 & 4:

extension String {
    var digits: String {
        return components(separatedBy: CharacterSet.decimalDigits.inverted)
            .joined()
    }
}