Pop3协议之三（编码）

取材自csdn文章 http://dev.csdn.net/develop/article/18/18448.shtm

通过pop3协议传输的主要是可打印字符ascii，其他编码比如unicode需要经过编码，通常的编码方式是Base64（=?*****? B）和Quoted-printable(=?******? Q) 其中*代表字符集

示例

Subject:=?gb2312?B?KMjw0MfM4cq+LbTL08q8/r/JxNzKx8Csu/jTyrz+KQ==?==?GB2312?B?1vfM4qO6?=

Subject:在邮件头，表示本行为邮件主题

=?gb2312? B?    表明后面一直到下一个?=出现，中间的字符串是通过 base64编码的

Subject: =?GB2312?Q?=D0=A1=F6=A9=B5=D8=D6=B7?=

同理，=?GB2312? Q?   一直到后面的 ?= 之间都是通过 Quoted-printable编码

具体如何编码可以看上面的文章，现在说解码

         private   static   string  ConvertCharSet( string  source)
        {
             string  result  =  source;
            Regex regex  =   new  Regex( @" =\?([a-zA-Z0-9\-]*)\?B\?([a-zA-Z0-9\+\/=]*)\?= " ,RegexOptions.IgnoreCase);
            Match m  =  regex.Match(source);
             while (m.Success)
            {
                result  =  regex.Replace(result ,System.Text.Encoding.GetEncoding( " GB2312 " ).GetString(
                    System.Convert.FromBase64String(m.Groups[ 2 ].Value)));
                m = m.NextMatch();
            }
            regex  =   new  Regex( @" =\?([a-zA-Z0-9\-]*)\?Q\?([a-zA-Z0-9\+\/=]*)\?= " ,RegexOptions.IgnoreCase);
            m  =  regex.Match(result);
             while  (m.Success)
            {
                result  =  Common.FromQuote(m.Groups[ 2 ].Value);
                m  =  m.NextMatch();
            }
             return  result;
        }

Class Common

         public   static   string  FromQuote( string  data)
        {
             string  result  =  data;

            Regex regex1  =   new  Regex( " ((=[a-fA-F0-9]{2})+) " );
            Match m1  =  regex1.Match(data);
             while (m1.Success)
            {
                 string  temp  =  m1.Groups[ 1 ].Value;
                Regex regex2  =   new  Regex( " =([a-fA-F0-9]{2}) " );
                Match m2  =  regex2.Match(temp);
                 string  str  =   string .Empty;
                 byte [] buffer  =   new   byte [ 0 ];
                 while  (m2.Success)
                {
                
                     byte [] bs  =  buffer;
                     byte  b  =   byte .Parse(m2.Groups[ 1 ].Value.ToLower(),System.Globalization.NumberStyles.HexNumber);
                    buffer  =   new   byte [buffer.Length + 1 ];
                    bs.CopyTo(buffer, 0 );
                    buffer[buffer.Length - 1 ]  =  b;

                    m2 = m2.NextMatch();
                }
                temp  =  System.Text.Encoding.Default.GetString(buffer);
                result  =  regex1.Replace(result,temp, 1 );
                m1  =  regex1.Match(result);
            }
             return  result;
        }