DnsClient.NET | simple yet very powerful and high performant open source | DNS library

You need to change your regex expression from

var record = Regex.Match(nameServer.ToString(), @"ns\d"+ "."+ item).Value;

to:

var record = Regex.Match(nameServer.ToString(), @"ns\d+"+ "."+ item).Value;

When you add a plus symbol to \d, you are looking for one or unlimited digits.

You can use Regex101 for checking your regular expressions.

I am a bit surprised that this solution didn't help you, so I tried to fix your code.

In my example, the input was google.es, so in the following code: foreach (var item in domains), item=="google.es" and the DNS answer was: