using System;

using System.Text.RegularExpressions;                   

public class Program

{

    public static void Main()

    {

        string input = "www.google.com and http://www.google.com";

        string pattern = @"\bwww.google.com\b";

        string replace = "text.com";

        var regex = new Regex(pattern);

        string result = regex.Replace(input, replace, 1);

        Console.WriteLine(result);

    }

}