using System;

using System.Text.RegularExpressions;

​

string input = @"Line 1

Line 2

## Section 1

Line 3

## Section 2

Line 4";

​

string[] parts = Regex.Split(input, @"(?=^##)", RegexOptions.Multiline);

​

foreach (var part in parts)

{

    Console.WriteLine("--- Part ---");

    Console.WriteLine(part);

}