using System;

public class Program

{

    public static void Main()

    {

        string myString = "This is STRING a222 END, and this is STRING b2838 END.";

        //Jump to starting index of `STRING`

        for(int i = myString.IndexOf("STRING");i > 0; i = myString.IndexOf("STRING", i+1))

        {

            //Get Index of Each END

            var endIndex = myString.Substring(i + "STARTING".Length).IndexOf("END");

            //PRINT substring between STRING and END of each occurance

            Console.WriteLine(myString.Substring(i + "STARTING".Length-1, endIndex));

        }

    }

}