using System;

using System.Collections.Generic;



public static class StringExtensions

{

    public static List<String> GetAllStringsBetween(this String Input, String Start, String End)

    {

        List<String> Return = new List<String>();

        Int32 Result = 0;

        Int32 LResult = 0;

        while ((Result = Input.GetOffsetBetween(Start, End, Result)) != -1)

        {

            Return.Add(Input.GetStringBetween(Start, End, LResult));

            LResult = Result;

        }

        return Return;

    }



    public static String GetStringBetween(this String Input, String Start, String End)

    {

        return Input.GetStringBetween(Start, End, 0);

    }



    public static String GetStringBetween(this String Input, String Start, String End, Int32 Offset)

    {

        if (Offset < Input.Length)

        {

            Int32 StartPos = (Input.IndexOf(Start, Offset) + Start.Length);

            if (StartPos > -1)

            {

                Int32 EndPos = Input.IndexOf(End, StartPos);

                if (EndPos > -1)

                {

                    return Input.Substring(StartPos, (EndPos - StartPos));

                }

            }

        }

        return String.Empty;

    }



    private static Int32 GetOffsetBetween(this String Input, String Start, String End, Int32 Offset)

    {

        if (Offset < Input.Length)

        {

            Int32 StartPos = Input.IndexOf(Start, Offset);

            if (StartPos > -1)

            {

                Int32 EndPos = Input.IndexOf(End, (StartPos + Start.Length));

                if (EndPos > -1)

                {

                    return EndPos;

                }

            }

        }

        return -1;

    }

}


Extends the string class.

Example usage;
String test = "test";
String result = test.GetStringBetween("t", "t");

result == "es"