using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string text = "<a href='https://mashinchi.ir/hello' target='_blank'>hello</a>";

        string pattern = @"<a[\s]+[^>]*?href[\s]?=[\s\""\']+((?!https://bama.ir)(?!https://mashinchi.ir).*?)[\""\']+(.*?)>";

        string replacement = @"<a rel=""nofollow"" href=""$1""$2>";

        string result = Regex.Replace(text, pattern, replacement, RegexOptions.IgnoreCase);

        Console.WriteLine(result);

    }

}