using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.IO;
public class Test
{
	// Demo: https://r...content-available-to-author-only...1.com/r/M9iGUO/2
	public static readonly Regex reg = new Regex(@"^h(\d+)\.+\s*(.+)", RegexOptions.Compiled | RegexOptions.Multiline); 
	
	public static void Main()
	{
		var inputText = "h1. Topic 1\r\nblah blah blah, because of bla bla bla\r\nh2. PartA\r\nblah blah blah\r\nh3. Part a\r\nblah blah blah\r\nh2. Part B\r\nblah blah blah\r\nh1. Topic 2\r\nand its cuz blah blah\r\nFIN";
		var res = ProcessHeadersInText(inputText, 2);
		Console.WriteLine(res);
	}
	public static string ProcessHeadersInText(string inputText, int atLevel = 1) 
	{
		return reg.Replace(inputText, m =>
			string.Format("<h{0}>{1}</h{0}>", (int.Parse(m.Groups[1].Value) > 9 ?
				9 : int.Parse(m.Groups[1].Value) + atLevel), m.Groups[2].Value.Trim()));
	}
}