WinApp C# ขอความช่วยเหลือเรื่อง RegEx ในการจำกัดไฟล์ หน่อยครับ

// Create a request for the URL.
        WebRequest request = WebRequest.Create("http://www.microsoft.com/en/us/default.aspx");
        // If required by the server, set the credentials.
        request.Credentials = CredentialCache.DefaultCredentials;
        // Get the response.
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        // Display the status.
        Console.WriteLine(response.StatusDescription);
        // Get the stream containing content returned by the server.
        Stream dataStream = response.GetResponseStream();
        // Open the stream using a StreamReader for easy access.
        StreamReader reader = new StreamReader(dataStream);
        // Read the content.
        string responseFromServer = reader.ReadToEnd();
        // Display the content.
        Console.WriteLine(responseFromServer);
        // Cleanup the streams and the response.
        reader.Close();
        dataStream.Close();
        response.Close();


        //reads the html into an html document to enable parsing
        IHTMLDocument2 doc = new HTMLDocumentClass();
        doc.write(new object[] { responseFromServer });
        doc.close();


        //loops through each element in the document to check if it qualifies for the attributes to be set
        foreach (IHTMLElement el in (IHTMLElementCollection)doc.all)
        {
            // check to see if all the desired attributes were found with the correct values
            bool qualify = true;
            if (el.tagName == "META")
            {
                HTMLMetaElement meta = (HTMLMetaElement)el;
                Response.Write("Content " + meta.content +"<br/>");
            }
           

        }