附录A ：收集的语法
这个附录中包含了本文档中定义的全部BNF语法。
对于自身来说，这些语法是完整的。其中提及了几个在RFC 822中定义的语法名称，而没有重复进行定义，这样可以降低由于无意识操作而在两种定义中产生差别的风险。一旦发现了未定义的术语，请参考RFC 822中的相关定义。
attribute := token
; Matching of attributes
; is ALWAYS case-insensitive.
composite-type := "message" / "multipart" / extension-token
content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; Matching of media type and subtype
; is ALWAYS case-insensitive.
description := "Content-Description" ":" *text
discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token
encoding := "Content-Transfer-Encoding" ":" mechanism
entity-headers := [ content CRLF ]
[ encoding CRLF ]
[ id CRLF ]
[ description CRLF ]
*( MIME-extension-field CRLF )
extension-token := ietf-token / x-token
hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used for characters > 127, =,
; SPACEs or TABs at the ends of lines, and is
; recommended for any character not listed in
; RFC 2049 as "mail-safe".
iana-token := <A publicly-defined extension token. Tokens
of this form must be registered with IANA
as specified in RFC 2048.>
ietf-token := <An extension token defined by a
standards-track RFC and registered
with IANA.>
id := "Content-ID" ":" msg-id
mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" /
ietf-token / x-token
MIME-extension-field := <Any RFC 822 header field which
begins with the string
"Content-">
MIME-message-headers := entity-headers
fields
version CRLF
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
MIME-part-headers := entity-headers
[fields]
; Any field not beginning with
; "content-" can have no defined
; meaning and may be ignored.
; The ordering of the header
; fields implied by this BNF
; definition should be ignored.
parameter := attribute "=" value
ptext := hex-octet / safe-char
qp-line := *(qp-segment transport-padding CRLF)
qp-part transport-padding
qp-part := qp-section
; Maximum length of 76 characters
qp-section := [*(ptext / SPACE / TAB) ptext]
qp-segment := qp-section *(SPACE / TAB) "="
; Maximum length of 76 characters
quoted-printable := qp-line *(CRLF qp-line)
safe-char := <any octet with decimal value of 33 through
60 inclusive, and 62 through 126>
; Characters not listed as "mail-safe" in
; RFC 2049 are also not recommended.
subtype := extension-token / iana-token
token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or tspecials>
transport-padding := *LWSP-char
; Composers MUST NOT generate
; non-zero length transport
; padding, but receivers MUST
; be able to handle padding
; added by message transports.
tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values
type := discrete-type / composite-type
value := token / quoted-string
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
