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(@"=\?.*?\?B\?.*?\?=", RegexOptions.IgnoreCase);
        var charSetMatches = charSetOccurences.Matches(msg);
        foreach (Match match in charSetMatches)
        {
            string charSet = match.Groups[0].Value.Replace("=?", "").Replace("?B?", "").Replace("?b?", "");
            Console.WriteLine(charSet);
        }
    }
}