C# Online Compiler | .NET Fiddle

using System;

public static class Program
{
		/// <summary>
        /// Define common patterns for substing actions
        /// </summary>
        public enum StrCut
        {
            BeforeLessStr = 0,
            BeforeInclStr = 1,
            AfterLessStr = 2,
            AfterInclStr = 4
        }
        /// <summary>
        /// Extend Susbstring with ability to pass a string and choose common actions surrounding that string.
        /// </summary>
        /// <param name="s"></param>
        /// <param name="arg">string argument</param>
        /// <param name="e">enum</param>
        /// <returns></returns>
        /// <author>Wed 01-May-19 - metadataconsult@gmail.com</author>
        public static string Substring(this string s, string arg, StrCut e)
        {
            //Range checking, why do you neet a unit test
            if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(arg)) return string.Empty;
            if (arg.Length > s.Length) return string.Empty;

            int idxarg = s.LastIndexOf(arg);

            if (idxarg == -1) return string.Empty; //is arg found in string

            bool argfits = (idxarg + arg.Length) <= s.Length; //is arg within limit of end of string
            
            if (e.Equals(StrCut.BeforeLessStr))
            {
                s = s.Substring(0, idxarg);
            }
            else if (argfits && e.Equals(StrCut.BeforeInclStr))
            {
                s = s.Substring(0, (idxarg + arg.Length));
            }
            else if (argfits && e.Equals(StrCut.AfterLessStr))
            {
                s = s.Substring(idxarg + arg.Length);
            }
            else if (e.Equals(StrCut.AfterInclStr))
            {
                s = s.Substring(idxarg);
            }
            return s;
        }
	
	public static void Main()
	{
		//string SourceURL = "SourceURL:https://metadataconsulting.blogspot.com/2019/04/Using-HTACG-HTML-Tidy-official-release-binaries-for-TidyHTML5Mananged.html"; 
		string SourceURL = "   {\"InsertSymbolText\",\"InsertSymbol\"},";  
		//string SURL = "SourceURL:";
		//string SURL = "SourceURL:https://metadataconsulting.blogspot.com/2019/04/Using-HTACG-HTML-Tidy-official-release-binaries-for-TidyHTML5Mananged.html"; 
		//string SURL = ".html";
		string SURL = ",";
		Console.WriteLine("Let's get rid of length param, like this Substring(0) = " + SourceURL.Substring(0)); 
				
		Console.WriteLine("BEGIN - Most of time, I just want to get a supply a string a work around it"); 
		Console.WriteLine("org = "+ SourceURL); 
		Console.WriteLine("AfterLessStr = " + SourceURL.Substring(SURL,StrCut.AfterLessStr));
		Console.WriteLine("AfterInclStr = " + SourceURL.Substring(SURL,StrCut.AfterInclStr));
		Console.WriteLine("BeforeInclStr = " + SourceURL.Substring(SURL,StrCut.BeforeInclStr));
		Console.WriteLine("BeforeLessStr = " + SourceURL.Substring(SURL,StrCut.BeforeLessStr));
		Console.WriteLine("END");
	}
}

        //string SourceURL = "SourceURL:https://metadataconsulting.blogspot.com/2019/04/Using-HTACG-HTML-Tidy-official-release-binaries-for-TidyHTML5Mananged.html"; 
 
using System;
​
public static class Program
{
        /// <summary>
        /// Define common patterns for substing actions
        /// </summary>
        public enum StrCut
        {
            BeforeLessStr = 0,
            BeforeInclStr = 1,
            AfterLessStr = 2,
            AfterInclStr = 4
        }
        /// <summary>
        /// Extend Susbstring with ability to pass a string and choose common actions surrounding that string.
        /// </summary>
        /// <param name="s"></param>
        /// <param name="arg">string argument</param>
        /// <param name="e">enum</param>
        /// <returns></returns>
        /// <author>Wed 01-May-19 - metadataconsult@gmail.com</author>
        public static string Substring(this string s, string arg, StrCut e)
        {

Cached Result

Let's get rid of length param, like this Substring(0) =    {"InsertSymbolText","InsertSymbol"},
BEGIN - Most of time, I just want to get a supply a string a work around it
org =    {"InsertSymbolText","InsertSymbol"},
AfterLessStr =
AfterInclStr = ,
BeforeInclStr =    {"InsertSymbolText","InsertSymbol"},
BeforeLessStr =    {"InsertSymbolText","InsertSymbol"}
END