using System;
using System.Linq;
using System.IO;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var content="heading1: contents with respect to heading1 heading2: heading2 contents heading3: heading 3 related contents sample strings";
		var delimiters = new string[] {"heading1:","heading2:","heading3:"};
        Console.WriteLine(
        	string.Join("\n", 
        			Regex.Split(content, $@"(?={string.Join("|", delimiters.Select(m => Regex.Escape(m)))})")
        				 .Where(x => !string.IsNullOrEmpty(x))
        		)
        	);
	}
}

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