原文:http://tools.ietf.org/html/rfc5234
Core Rules
Certain basic rules are in uppercase, such as SP,HTAB, CRLF, DIGIT,
ALPHA, etc.
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
BIT = "0" / "1"
CHAR = %x01-7F
; any 7-bitUS-ASCII character,
; excluding NUL
CR = %x0D
; carriagereturn
CRLF = CR LF
; Internet standardnewline
CTL = %x00-1F / %x7F
; controls
DIGIT = %x30-39
; 0-9
DQUOTE = %x22
; " (Double Quote)
HEXDIG = DIGIT / "A" / "B" /"C" / "D" / "E" / "F"
HTAB = %x09
; horizontaltab
LF = %x0A
; linefeed
LWSP = *(WSP / CRLF WSP)
; Use of thislinear-white-space rule
; permits lines containing only white
; space that are no longer legal in
; mail headers and have caused
; interoperability problems in other
; contexts.
; Do not usewhen defining mail
; headers and use with caution in
; other contexts.
OCTET = %x00-FF
; 8 bits ofdata
SP = %x20
VCHAR = %x21-7E
; visible(printing) characters
WSP = SP / HTAB
; white space