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)
        {
            string[] encoding = match.Groups[0].Value.Split(new string[]{ "?" }, StringSplitOptions.None);
            string charSet = encoding[1];
            string encodeType = encoding[2];
            string encodedString = encoding[3];
            Console.WriteLine("Charset: " + charSet);
            Console.WriteLine("Encoding type: " + encodeType);
            Console.WriteLine("Encoded String: " + encodedString + "\n");
        }
    }
}