using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string msg= "=?windows-1258?B?UkU6IFRyIDogUGxhbiBkZSBjb250aW51aXTpIGQnYWN0aXZpdOkgZGVz?= =?windows-1258?B?IHNlcnZldXJzIFdlYiBHb1ZveWFnZXN=?=";
        var charSetOccurences = new Regex(@"=\?(.*?)\?([BQ])\?(.*?)\?=", RegexOptions.IgnoreCase);
        MatchCollection matches = charSetOccurences.Matches(msg);
        foreach (Match match in matches)
        {
            Console.WriteLine("Charset: " + match.Groups[1].Value);
            Console.WriteLine("Encoding type: " + match.Groups[2].Value);
            Console.WriteLine("Encoded String: " + match.Groups[3].Value + "\n");
        }
    }
}