using System;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
	public static void Main()
	{
		string s1 = "<p>Hello world, hello world</p>";
		string[] terms = new string[] {"hello", "world"};
		var match = 1;
		s1 = Regex.Replace(s1,
		    String.Join("|", String.Join("|", terms.OrderByDescending(s => s.Length)
		        .Select(Regex.Escape))),
		    m => String.Format("<span id=\"m_{0}\">{1}</span>", match++, m.Value),
		    RegexOptions.IgnoreCase);
		Console.Write(s1);
	}
}