Appendix B. Core ABNF of ABNF

This appendix contains some basic rules that are in common use. Basic rules are in uppercase. Note that these rules are only valid for ABNF encoded in 7-bit ASCII or in characters sets that are a superset of 7-bit ASCII.

B.1. 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-bit US-ASCII character,
                             ;  excluding NUL

      CR             =  %x0D
                             ; carriage return

      CRLF           =  CR LF
                             ; Internet standard newline

      CTL            =  %x00-1F / %x7F
                             ; controls

      DIGIT          =  %x30-39
                             ; 0-9

      DQUOTE         =  %x22
                             ; " (Double Quote)

      HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

      HTAB           =  %x09
                             ; horizontal tab

      LF             =  %x0A
                             ; linefeed

      LWSP           =  *(WSP / CRLF WSP)
                             ; Use of this linear-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 use when defining mail
                             ;  headers and use with caution in
                             ;  other contexts.

      OCTET          =  %x00-FF
                             ; 8 bits of data

      SP             =  %x20

      VCHAR          =  %x21-7E
                             ; visible (printing) characters

      WSP            =  SP / HTAB
                             ; white space

B.2. Common Encoding

Externally, data are represented as "network virtual ASCII" (namely, 7-bit US-ASCII in an 8-bit field), with the high (8th) bit set to zero. A string of values is in "network byte order", in which the higher-valued bytes are represented on the left-hand side and are sent over the network first.