Get index from address

.cs

using System;

namespace GetIndexFromAddress
{
    class Program
    {
        static int? NextDigitIndex(string s, int start)
        {
            for (int i = start; i < s.Length; ++i)
            {
                if (char.IsDigit(s[i]))
                    return i;
                if (!char.IsWhiteSpace(s[i]))
                    return null;
            }
            return null;
        }

        static int? GetIndex(ref string address)
        {
            int indexEnd = -1;
            string indexString = "";
            for (int digit = 0; digit < 6; ++digit)
            {
                int? i = NextDigitIndex(address, indexEnd + 1);
                if (i == null)
                    return null;
                indexString += address[(int) i];
                indexEnd = (int) i;
            }

            int index = int.Parse(indexString);

            address = address.Remove(0, indexEnd + 1).TrimStart(',', ' ');

            return index;
        }

        static void Main()
        {
            while (true)
            {
                string address = Console.ReadLine();
                int? index = GetIndex(ref address);
                Console.WriteLine($"index: '{index}'");
                Console.WriteLine($"address: '{address}'");
                Console.WriteLine();
            }
        }
    }
}