C# Online Compiler | .NET Fiddle

using System;using System.Diagnostics; 

public static class Extensions 
{
	/// <summary>
	/// Get a substring between two anchor strings, minimal span
	/// </summary>
	/// <param name="s">source string</param>
	/// <param name="from">search from end of this string</param>
	/// <param name="to">to beginning of this string, searching backwards, from end to start of s</param>
	/// <returns>a substring between from and to, maximal span</returns>
	public static string GetFirstStringBetweenStringsMinSpanCleanup(this string s, string from, string to)
	{
		if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(from) || string.IsNullOrEmpty(to)) return string.Empty;

		int idxFrom = s.IndexOf(from);
		int idxStart = idxFrom + from.Length; //we filter "not found" -1, never race condtn

		if (idxFrom == -1 || idxStart >= s.Length - 1)
			return string.Empty;

		int idxEnd = s.IndexOf(to, idxStart); //Exact definition, but intuitively next line meets likely expectations -> YOU CHOOSE
        //int idxEnd = s.IndexOf(to, idxStart + 1); //Start next position after, leaving a space for 1 character to be returned

           
		if (idxEnd == -1 || idxEnd <= idxStart)
			return string.Empty;

		return s.Substring(idxStart, idxEnd - idxStart);

	}
	/// <summary>
	/// Get a substring between two anchor strings, MINIMAL SPAN
	/// </summary>
	/// <param name="s">source string</param>
	/// <param name="from">search from end of this string</param>
	/// <param name="to">to beginning of this string, searching backwards, from end to start of s</param>
	/// <returns>a substring between from and to, maximal span</returns>
	public static string GetFirstStringBetweenStringsMinSpan(this string s, string from, string to)
	{
		Console.Write("args from {0} to {1} = ", from, to); //debug

		//edge cases
		if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(from) || string.IsNullOrEmpty(to)) return string.Empty;
		//if (from.Length >= s.Length || to.Length >= s.Length) return string.Empty; //redundant, this Rule take care of below

		int idxFrom = s.IndexOf(from);
		int idxStart = idxFrom + from.Length; //we filter "not found" -1, never race condtn

		if (idxFrom == -1)       
			return string.Empty;
		else if (idxStart >= s.Length - 1) //for testing combine to 1 line, we combining a idx with a length, and w/ lengths we normally subtract 1 
		{
			Console.WriteLine("r1. idxStart={0} >= (s.Length - 1)={1}", idxStart, s.Length - 1);
			return string.Empty; 
		}

		int idxEnd = s.IndexOf(to, idxStart); //Exact definition, but intuitively next line meets likely expectations -> YOU CHOOSE
        //int idxEnd = s.IndexOf(to, idxStart + 1); //Start next position after, leaving a space for 1 character to be returned

		if (idxEnd == -1 )
			return string.Empty;
		else if (idxEnd > s.Length - 1) //Rule 2 never gets used
		{
			Console.WriteLine("r2. idxEnd={0} > (s.Length - 1)={1}", idxEnd, s.Length - 1);
			return string.Empty;
		}
		else if (idxEnd <= idxStart)
		{
			Console.WriteLine("r3. idxEnd={0} <= idxStart={1}", idxEnd, idxStart);
			return string.Empty;
		}

		return s.Substring(idxStart, idxEnd - idxStart);

	}
 }

public class Program
{
	public static void Main()
	{
		  string test = "abcd";
            Console.WriteLine(test);
            Console.WriteLine("=================");

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(null, null));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("", ""));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, test));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("", test));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test,""));
            
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "d"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "e"));
            
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("e", "e"));
            
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "d"));
            
            

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a","abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("b", "abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("c", "abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("d", "abc"));

            Console.WriteLine(); 
            test = "abcdabcd";
            Console.WriteLine(test);
            Console.WriteLine("=================");

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(null, null));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("", ""));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, test));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("", test));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, ""));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "d"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "e"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("e", "e"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("ab", "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "b"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "c"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan(test, "d"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("b", "abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("c", "abc"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("d", "abc"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("abc", "d")); //pass


            Console.WriteLine();
            test = "aaaa";
            Console.WriteLine(test);
            Console.WriteLine("=================");

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("aa", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("aaa", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("aaaa", "a"));

            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "a"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "aa"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "aaa"));
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("a", "aaaa"));

            Console.WriteLine();
            test = "aaaaaaaaaaaaaaaaaaaaaaaaaaaabbbb";
            Console.WriteLine(test);
            Console.WriteLine("=================");
            Console.WriteLine(test.GetFirstStringBetweenStringsMinSpan("bbbb", "aaaa"));

            string result = string.Empty; 
            Stopwatch sw = new Stopwatch();
            sw.Start();
            result = test.GetFirstStringBetweenStringsMinSpan("aaaaaaa", "bb");
            sw.Stop();
            Console.WriteLine(result);
            Console.WriteLine(" in " + sw.ElapsedTicks + " ticks."); 
	}
}

 
using System;using System.Diagnostics; 
​
public static class Extensions 
{
    /// <summary>
    /// Get a substring between two anchor strings, minimal span
    /// </summary>
    /// <param name="s">source string</param>
    /// <param name="from">search from end of this string</param>
    /// <param name="to">to beginning of this string, searching backwards, from end to start of s</param>
    /// <returns>a substring between from and to, maximal span</returns>
    public static string GetFirstStringBetweenStringsMinSpanCleanup(this string s, string from, string to)
    {
        if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(from) || string.IsNullOrEmpty(to)) return string.Empty;
​
        int idxFrom = s.IndexOf(from);
        int idxStart = idxFrom + from.Length; //we filter "not found" -1, never race condtn
​
        if (idxFrom == -1 || idxStart >= s.Length - 1)
            return string.Empty;
​
        int idxEnd = s.IndexOf(to, idxStart); //Exact definition, but intuitively next line meets likely expectations -> YOU CHOOSE
        //int idxEnd = s.IndexOf(to, idxStart + 1); //Start next position after, leaving a space for 1 character to be returned
​