Format phone number for each UITextField on a ViewController

Phone.swift

// https://stackoverflow.com/questions/1246439/uitextfield-for-phone-number/13227608#13227608

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool
    {
        var fullString = textField.text ?? ""
        fullString.append(string)
        if range.length == 1 {
            textField.text = format(phoneNumber: fullString, shouldRemoveLastDigit: true)
        } else {
            textField.text = format(phoneNumber: fullString)
        }
        return false
    }
    
    func format(phoneNumber: String, shouldRemoveLastDigit: Bool = false) -> String {
        guard !phoneNumber.isEmpty else { return "" }
        guard let regex = try? NSRegularExpression(pattern: "[\\s-\\(\\)]", options: .caseInsensitive) else { return "" }
        let r = NSString(string: phoneNumber).range(of: phoneNumber)
        var number = regex.stringByReplacingMatches(in: phoneNumber, options: .init(rawValue: 0), range: r, withTemplate: "")
        
        if number.count > 10 {
            let tenthDigitIndex = number.index(number.startIndex, offsetBy: 10)
            number = String(number[number.startIndex..<tenthDigitIndex])
        }
        
        if shouldRemoveLastDigit {
            let end = number.index(number.startIndex, offsetBy: number.count-1)
            number = String(number[number.startIndex..<end])
        }
        
        if number.count < 7 {
            let end = number.index(number.startIndex, offsetBy: number.count)
            let range = number.startIndex..<end
            number = number.replacingOccurrences(of: "(\\d{3})(\\d+)", with: "($1) $2", options: .regularExpression, range: range)
            
        } else {
            let end = number.index(number.startIndex, offsetBy: number.count)
            let range = number.startIndex..<end
            number = number.replacingOccurrences(of: "(\\d{3})(\\d{3})(\\d+)", with: "($1) $2-$3", options: .regularExpression, range: range)
        }
        
        return number
    }