decoding problem in email

static Encoding _encoding = Encoding.ASCII;
static string MatchEvaluatorHexToString(System.Text.RegularExpressions.Match m)
{
    // Get the matched string.  
    string str = m.ToString();
    Byte[] b = new Byte[1];
    b[0] = byte.Parse(m.Groups[1].Value.Replace("=", ""), System.Globalization.NumberStyles.HexNumber);
    string str4 = _encoding.GetString(b);
    return str.Replace(m.Groups[1].ToString(), str4);
}

static string MatchEvaluatorMIMEEnc2Str(System.Text.RegularExpressions.Match m)
{
    _encoding = Encoding.GetEncoding(m.Groups[1].Value);
    if (m.Groups[2].Value == "B")
    {
        Decoder sDecoder = _encoding.GetDecoder();
        byte[] todecodeByte = Convert.FromBase64String(m.Groups[3].Value);
        int charCount = sDecoder.GetCharCount(todecodeByte, 0, todecodeByte.Length);
        char[] decodedChar = new char[charCount];
        sDecoder.GetChars(todecodeByte, 0, todecodeByte.Length, decodedChar, 0);
        string result = new String(decodedChar);
        return (result);
    }
    System.Text.RegularExpressions.Regex regexItems = new System.Text.RegularExpressions.Regex("(=[0-9a-fA-F]{2})");
    string str6 = regexItems.Replace(m.Groups[3].Value.Replace("_", " "), new System.Text.RegularExpressions.MatchEvaluator(MatchEvaluatorHexToString));
    return str6;
}

static public string Decode(string str)
{
    System.Text.RegularExpressions.Regex regEx = new System.Text.RegularExpressions.Regex("=\\?(.+?)\\?(.+?)\\?(.+?)\\?=");
    System.Text.RegularExpressions.MatchCollection mc = regEx.Matches(str);
    return regEx.Replace(str, new System.Text.RegularExpressions.MatchEvaluator(MatchEvaluatorMIMEEnc2Str));
}

String decodedSubject=Decode("=?utf-8?B?RnJvbSBSZWRpZmY=?=");