C++ I need just a function that will take a constant reference to a string and will return a string. The function should look through the string and return any repeated sequence of 4 digits. If there isn't a repeated sequence of 4 digits exact, return a empty string.

Ex)

If the inputted string is "397390362927390", the function should return "7390" as it is repeated twice. At most there will be one repeated sequence in the input.