using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var data = new string[] {"trabc@abtrec.com.lo", "demomail@demodomain.com"};
		foreach (var s in data) {
			var res = Regex.Replace(
				s
			,   @"^.*(?=.{2}\@.{2})|(?<=.{2}\@.{2}).*(?=.com.*$)"
			,   match => new string('*', match.ToString().Length)
			);
			Console.WriteLine("'{0}' ---> '{1}'", s, res);
		}
	}
}