Credit cards number formatting

String+Extension.swift

extension String {
    func insertStringAt(indexes: [Int], string: String) -> String {
        var formatted = String()
        for (i, str) in self.enumerated() {
            if indexes.contains(i) {
                formatted.append("\(str)\(string)")
            } else {
                formatted.append(str)
            }
        }
        return formatted
    }
}

/*
Eg: "0000000000000000".insertStringAt(indexes: [3, 7, 11], string: " ")
output: "0000 0000 0000 0000"
*/