using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
    	var s = "Hello\nabc\nHello\ndef\nHello";
    	var i = 0;
    	var result = Regex.Replace(s, "Hello", m => string.Format("{0}{1}",m.Value,++i), RegexOptions.IgnoreCase);
       	Console.WriteLine(result);
	}
}