如何理解IMAP電子郵件文本中的等號'='符號？

Question

我目前使用Python imaplib處理電子郵件文本。

我使用fetch命令從GMail服務器獲取原始數據電子郵件。但是，我發現一件事情非常棘手 - 等號'='。這不是一個正常的等號，而是一個特殊的符號。

例如：

1. '=' 有時充當斷字標記在文本行的末尾：

   Depending upon your module selections, course lecturers may also contact yo= 
   u with preparatory work over the next few weeks. It would be wise to start = 
   reviewing the preparatory reading lists provided on the module syllabi now = 
   

2. 有時，它充當類似於「％一逃逸標記」，例如：

   a=20b實際上是a<SPACE>b
=46rom here是動作ually From here

我完全對這種奇怪的符號感到困惑。我認爲必須有一個指導來處理這個問題，因爲GMail可以在他們的應用程序中正確處理這些事情。

我看到這與HTML編碼有關，就像'％'將被編碼。但問題是，我從IMAP響應得到的是一個包含'='符號的字符串。我該如何處理？使用正則表達式？

Answer 1

簡而言之，一行末尾的等號表示軟換行符。等號後跟兩個十六進制字符（0-9，A-F）編碼一個八位字節（字節）。

該編碼方案被稱爲「引用打印」，並在RFC 2045的6.7節中定義。特別參見項目（1）和（5）。

6.7. Quoted-Printable Content-Transfer-Encoding 

    The Quoted-Printable encoding is intended to represent data that 
    largely consists of octets that correspond to printable characters in 
    the US-ASCII character set. It encodes the data in such a way that 
    the resulting octets are unlikely to be modified by mail transport. 
    If the data being encoded are mostly US-ASCII text, the encoded form 
    of the data remains largely recognizable by humans. A body which is 
    entirely US-ASCII may also be encoded in Quoted-Printable to ensure 
    the integrity of the data should the message pass through a 
    character-translating, and/or line-wrapping gateway. 

    In this encoding, octets are to be represented as determined by the 
    following rules: 

    (1) (General 8bit representation) Any octet, except a CR or 
      LF that is part of a CRLF line break of the canonical 
      (standard) form of the data being encoded, may be 
      represented by an "=" followed by a two digit 
      hexadecimal representation of the octet's value. The 
      digits of the hexadecimal alphabet, for this purpose, 
      are "ABCDEF". Uppercase letters must be 
      used; lowercase letters are not allowed. Thus, for 
      example, the decimal value 12 (US-ASCII form feed) can 
      be represented by "=0C", and the decimal value 61 (US- 
      ASCII EQUAL SIGN) can be represented by "=3D". This 
      rule must be followed except when the following rules 
      allow an alternative encoding. 

    (2) (Literal representation) Octets with decimal values of 
      33 through 60 inclusive, and 62 through 126, inclusive, 
      MAY be represented as the US-ASCII characters which 
      correspond to those octets (EXCLAMATION POINT through 
      LESS THAN, and GREATER THAN through TILDE, 
      respectively). 

    (3) (White Space) Octets with values of 9 and 32 MAY be 
      represented as US-ASCII TAB (HT) and SPACE characters, 

      respectively, but MUST NOT be so represented at the end 
      of an encoded line. Any TAB (HT) or SPACE characters 
      on an encoded line MUST thus be followed on that line 
      by a printable character. In particular, an "=" at the 
      end of an encoded line, indicating a soft line break 
      (see rule #5) may follow one or more TAB (HT) or SPACE 
      characters. It follows that an octet with decimal 
      value 9 or 32 appearing at the end of an encoded line 
      must be represented according to Rule #1. This rule is 
      necessary because some MTAs (Message Transport Agents, 
      programs which transport messages from one user to 
      another, or perform a portion of such transfers) are 
      known to pad lines of text with SPACEs, and others are 
      known to remove "white space" characters from the end 
      of a line. Therefore, when decoding a Quoted-Printable 
      body, any trailing white space on a line must be 
      deleted, as it will necessarily have been added by 
      intermediate transport agents. 

    (4) (Line Breaks) A line break in a text body, represented 
      as a CRLF sequence in the text canonical form, must be 
      represented by a (RFC 822) line break, which is also a 
      CRLF sequence, in the Quoted-Printable encoding. Since 
      the canonical representation of media types other than 
      text do not generally include the representation of 
      line breaks as CRLF sequences, no hard line breaks 
      (i.e. line breaks that are intended to be meaningful 
      and to be displayed to the user) can occur in the 
      quoted-printable encoding of such types. Sequences 
      like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely 
      appear in non-text data represented in quoted- 
      printable, of course. 

      Note that many implementations may elect to encode the 
      local representation of various content types directly 
      rather than converting to canonical form first, 
      encoding, and then converting back to local 
      representation. In particular, this may apply to plain 
      text material on systems that use newline conventions 
      other than a CRLF terminator sequence. Such an 
      implementation optimization is permissible, but only 
      when the combined canonicalization-encoding step is 
      equivalent to performing the three steps separately. 

    (5) (Soft Line Breaks) The Quoted-Printable encoding 
      REQUIRES that encoded lines be no more than 76 
      characters long. If longer lines are to be encoded 
      with the Quoted-Printable encoding, "soft" line breaks 
      must be used. An equal sign as the last character on a 
      encoded line indicates such a non-significant ("soft") 
      line break in the encoded text.