using System;
using System.Text.RegularExpressions;
 
namespace RegExApplication
{
   public class Program
   {
      public static void Main(string[] args)
      {
        
         var pattern = "<<.*?FirstName.*?>>";
  	var template = "<<FirstName>> <<FirstName>>";
  	var replaceWith = "FOO";

  	var regex = new Regex(pattern); 

  		Console.WriteLine(regex.Replace(template, replaceWith));
      }
   }
}