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Appendix B. Core ABNF of ABNF

B.1. Core Rules​

Certain basic rules are in uppercase, such as SP, HTAB, CRLF, DIGIT, ALPHA, etc. These core rules are defined in this appendix.

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.

Character Encoding​

The core rules use US-ASCII encoding. Each character is represented by its corresponding ASCII value:

  • Letters (A-Z, a-z): %x41-5A, %x61-7A
  • Digits (0-9): %x30-39
  • Common symbols: SP (%x20), HTAB (%x09), etc.

Usage Guidelines​

These core rules are designed to be used across multiple Internet specifications. They provide a common vocabulary for basic syntactic elements, ensuring consistency across different protocols and formats.

Important Notes:

  • LWSP (linear white space) should be used with caution, as it permits lines containing only white space
  • The core rules assume a 7-bit US-ASCII environment
  • For specifications using different character encodings, appropriate adjustments should be made
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