login_cmd=&login_params=&login_email=my%40mail.de&login_password=TOPsecret&submit..........

my%40mail.de

TOPsecret

string param1 = HttpUtility.ParseQueryString(myUri.Query).Get("login_email");

(?<=login_email=)[^&]+|(?<=login_password=)[^&]+

(?<=login_(?:email|password)=)[^&]*(?=&)

(?<=login_(?:email|password)=)[^&]*

String input = @"login_cmd=&login_params=&login_email=my%40mail.de&login_password=TOPsecret&submit..........";
Regex rgx = new Regex(@"(?<=login_(?:email|password)=)[^&]*");
foreach (Match m in rgx.Matches(input))
Console.WriteLine(m.Groups[0].Value);

String input = @"login_cmd=&login_params=&login_email=my%40mail.de&login_password=TOPsecret&submit..........";

Regex rgx = new Regex(@"(?<=login_email=)([^&]+)|(?<=login_password=)([^&]+)");

foreach (Match m in rgx.Matches(input))
{
    if(!m.Groups[1].Value.ToString().Equals(string.Empty))
    {
        Console.WriteLine("Email : " + m.Groups[1].Value);
    }
        if(!m.Groups[2].Value.ToString().Equals(string.Empty))
    {
        Console.WriteLine("Password : " + m.Groups[2].Value);
    }
}