Regex regex = new Regex(@"code=(S*)&state");

MatchesCollections matches = regex.Matches(decodeMessage);

var decodeMessage = "GET /?code=KYTW/9Bjv7m7OhrXmuZd50PuvRq5b4TVi3GIH5ZXOVl59XQL8yMAhuC2IqS7XIsxSQauZA" +
"==&state= HTTP/1.1";
var regex = new Regex(@"code=(S*)&state");

var matches = regex.Matches(decodeMessage);
foreach (Match element in matches)
{
    Console.WriteLine(element.Groups[1].Value);
}