/**
      Format the current string into the desired pattern.
      - Parameters:
        - mask: A format of the mask. e.g. "XXX-XXXX"
      - Returns: A result string after formatted.
     */
    func format(mask: String) -> String {
        let numbers = replacingOccurrences(of: "[^0-9]", with: "", options: .regularExpression)
        var result = ""
        var index = numbers.startIndex // numbers iterator

        // iterate over the mask characters until the iterator of numbers ends
        for ch in mask where index < numbers.endIndex {
            if ch == "X" {
                // mask requires a number in this place, so take the next one
                result.append(numbers[index])

                // move numbers iterator to the next index
                index = numbers.index(after: index)

            } else {
                result.append(ch) // just append a mask character
            }
        }
        return result
    }