redirects domain.com to www.domain.com

regex-domain-to-www-domain.cs

if (Regex.IsMatch(Request.Url.AbsoluteUri, @"^((http|https)://[^www]).*$"))
{
    string newDomain = String.Format("{0}://www.{1}{2}",
                                     Request.Url.Scheme, 
                                     Request.Url.Authority, 
                                     Request.Url.AbsolutePath);

    Response.Status = "301 Moved Permanently";
    Response.AddHeader("Location", newDomain);
    Response.End();
}

Eu faria assim

var pattern = @"^(https?://)(?!www[.])(.+)$";
var url = Regex.Replace(Request.Url.AbsoluteUri, pattern, @"$1www.$2");

Response.Status = "301 Moved Permanently";
Response.AddHeader("Location", url);
Response.End();

Com a regex atual: ^((http|https)://[^www]).*$
Um domínio assim: http://wordpress.com não daria um match
Detalhe o [] significa qual caracteres serão permitidos, e usando ^ como primeiro caractere dele vai ser negando os caracteres, então:
[w] == [www] => true
[^w] == [^www] => true