using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string input = "here is my substring and then somestring";

​

        int position;

        if (TryGetIndex(input, "(?<=my substring.*)somestring", out position)){

            Console.WriteLine($"somestring index: {position}");

        }

        else if (TryGetIndex(input, "(?<=other substring.*)otherstring", out position)) {

            Console.WriteLine($"otherstring index: {position}");

        }

​

        bool TryGetIndex(string input, string pattern, out int position){

            var match = Regex.Match(input, pattern);

            if (match.Success){

                position = match.Index;

                return true;

            }

            position = -1;

            return false;

        }

    }

}