Formatting Phone number in Swift

Masked number typing

/// mask example: `+X (XXX) XXX-XXXX`
func format(with mask: String, phone: String) -> String {
    let numbers = phone.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
}

Call the above function from the UITextField delegate method:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    guard let text = textField.text else { return false }
    let newString = (text as NSString).replacingCharacters(in: range, with: string)
    textField.text = format(with: "+X (XXX) XXX-XXXX", phone: newString)
    return false
}

So, that works better.

"" => ""
"0" => "+0"
"412" => "+4 (12"
"12345678901" => "+1 (234) 567-8901"
"a1_b2-c3=d4 e5&f6|g7h8" => "+1 (234) 567-8"