MIME extersion part 2:
Message Header Extensions for Non-ASCII Text
Here is a brief explanation:
编码规则:
encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
charset = token ; see section 3
encoding = token ; see section 4
token = 1*<Any CHAR except SPACE, CTLs, and especials>
especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
<"> / "/" / "[" / "]" / "?" / "." / "="
encoded-text = 1*<Any printable ASCII character other
than "?" or SPACE>
; (but see "Use of encoded-words in message
; headers", section 5)
Both "encoding" and "charset" names are case-independent. Thus the
charset name "ISO-8859-1" is equivalent to "iso-8859-1", and the
encoding named "Q" may be spelled either "Q" or "q".
4. Encodings
Initially, the legal values for "encoding" are "Q" and "B". These
encodings are described below. The "Q" encoding is recommended for
use when most of the characters to be encoded are in the ASCII
character set; otherwise, the "B" encoding should be used.
Nevertheless, a mail reader which claims to recognize encoded-words
MUST be able to accept either encoding for any character set which it
supports.
4.1. The "B" encoding
The "B" encoding is identical to the "BASE64" encoding defined by RFC
1521.
4.2. The "Q" encoding
The "Q" encoding is similar to the "Quoted-Printable" content-
transfer-encoding defined in RFC 1521. It is designed to allow text
containing mostly ASCII characters to be decipherable on an ASCII
terminal without decoding.
(1) Any 8-bit value may be represented by a "=" followed by two
hexadecimal digits. For example, if the character set in use
were ISO-8859-1, the "=" character would thus be encoded as
"=3D", and a SPACE by "=20". (Upper case should be used for
hexadecimal digits "A" through "F".)
(2) The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be
represented as "_" (underscore, ASCII 95.). (This character may
not pass through some internetwork mail gateways, but its use
will greatly enhance readability of "Q" encoded data with mail
readers that do not support this encoding.) Note that the "_"
always represents hexadecimal 20, even if the SPACE character
occupies a different code position in the character set in use.
(3) 8-bit values which correspond to printable ASCII characters other
than "=", "?", "_" (underscore), and SPACE may be represented as
those characters. (But see section 5 for restrictions.)
