RFC 2234

Augmented Backus–Naur Form

In computer science, Augmented Backus–Naur Form (ABNF) is a metalanguage based on Backus–Naur Form (BNF), but consisting of its own syntax and derivation rules. The motive principle for ABNF is to describe a formal system of a language to be used as a bidirectional communications protocol. It is defined by Internet Standard 68 (STD 68), which as of May 2008 is RFC 5234, and it often serves as the definition language for IETF communication protocols.

RFC 5234 supersedes RFC 4234 (which superseded RFC 2234).


An ABNF specification is a set of derivation rules, written as

rule = definition ; comment CR LF

where rule is a case-sensitive nonterminal, the definition consists of sequences of symbols that define the rule, a comment for documentation, and ending with a carriage return and line feed.

Rule names are case insensitive: , , , and all refer to the same rule. Rule names consist of a letter followed by letters, numbers, and hyphens.

Angle brackets (“<”, “>”) are not required around rule names (as they are in BNF). However they may be used to delimit a rule name when used in prose to discern a rule name.

ABNF is encoded in ASCII (which is seven bits) in an eight-bit field with the high bit set to zero.

Terminal values

Terminals are specified by one or more numeric characters.

Numeric characters may be specified as the percent sign “%”, followed by the base (b = binary, d = decimal, and x = hexadecimal), followed by the value, or concatenation of values (indicated by “.”). For example a carriage return is specified by %d13 in decimal or %x0D in hexadecimal. A carriage return followed by a line feed may be specified with concatenation as %d13.10.

Literal text is specified through the use of a string enclosed in quotation marks ("). These strings are case-insensitive and the character set used is (US-)ASCII. Therefore the string “abc” will match “abc”, “Abc”, “aBc”, “abC”, “ABc”, “AbC”, “aBC”, and “ABC”. For a case-sensitive match the explicit characters must be defined: to match “aBc” the definition will be %d97 %d66 %d99.


White space

White space is used to separate elements of a definition; for space to be recognized as a delimiter it must be explicitly included.


Rule1 Rule2

A rule may be defined by listing a sequence of rule names.

To match the string “aba” the following rules could be used:

  1. fu = %x61 ; a
  2. bar = %x62 ; b
  3. mumble = fu bar fu


Rule1 / Rule2

A rule may be defined by a list of alternative rules separated by a solidus ("/").

To accept the rule fu or the rule bar the following rule could be constructed:

  1. fubar = fu / bar

Incremental alternatives

Rule1 =/ Rule2

Additional alternatives may be added to a rule through the use of “=/” between the rule name and the definition.

The rule

  1. ruleset = alt1 / alt2 / alt3 / alt4 / alt5

is equivalent to

  1. ruleset = alt1 / alt2
  2. ruleset =/ alt3
  3. ruleset =/ alt4 / alt5

Value range


A range of numeric values may be specified through the use of a hyphen (“-”).

The rule

  1. OCTAL = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"

is equivalent to

  1. OCTAL = %x30-37

Sequence group

(Rule1 Rule2)

Elements may be placed in parentheses to group rules in a definition.

To match “elem fubar snafu” or “elem tarfu snafu” the following rule could be constructed:

  1. group = elem (fubar / tarfu) snafu

To match “elem fubar” or “tarfu snafu” the following rules could be constructed:

  1. group = elem fubar / tarfu snafu
  2. group = (elem fubar) / (tarfu snafu)

Variable repetition


To indicate repetition of an element the form <a>*<b>element is used. The optional <a> gives the minimum number of elements to be included with the default of 0. The optional <b> gives the maximum number of elements to be included with the default of infinity.

Use *element for zero or more elements, 1*element for one or more elements, and 2*3element for two or three elements.

Specific repetition


To indicate an explicit number of elements the form <a>element is used and is equivalent to <a>*<a>element.

Use 2DIGIT to get two numeric digits and 3DIGIT to get three numeric digits. (DIGIT is defined below under 'Core rules'. Also see zip-code in the example below.)

Optional sequence


To indicate an optional element the following constructions are equivalent:

  1. [fubar snafu]
  2. *1(fubar snafu)
  3. 0*1(fubar snafu)


; comment

A semicolon (“;”) starts a comment that continues to the end of the line.

Operator precedence

The following operators have the given precedence from tightest binding to loosest binding:

  1. Strings, Names formation
  2. Comment
  3. Value range
  4. Repetition
  5. Grouping, Optional
  6. Concatenation
  7. Alternative

Use of the alternative operator with concatenation may be confusing and it is recommended that grouping be used to make explicit concatenation groups.

Core rules

The core rules are defined in the ABNF standard.
Rule Formal Definition Meaning
ALPHA %x41-5A / %x61-7A Upper- and lower-case ASCII letters (A–Z, a–z)
DIGIT %x30-39 Decimal digits (0–9)
HEXDIG DIGIT / "A" / "B" / "C" / "D" / "E" / "F" Hexadecimal digits (0–9, A–F, a–f)
DQUOTE %x22 Double Quote
SP %x20 space
HTAB %x09 horizontal tab
WSP SP / HTAB space and horizontal tab
LWSP *(WSP / CRLF WSP) linear white space (past newline)
VCHAR %x21-7E visible (printing) characters
CHAR %x01-7F any ASCII character, excluding NUL
OCTET %x00-FF 8 bits of data
CTL %x00-1F / %x7F controls
CR %x0D carriage return
LF %x0A linefeed
CRLF CR LF Internet standard newline
BIT "0" / "1" binary digit


The postal address example given in the Backus–Naur Form (BNF) page may be specified as follows:
postal-address   = name-part street zip-part

name-part = *(personal-part SP) last-name [SP suffix] CRLF name-part =/ personal-part CRLF

personal-part = first-name / (initial ".") first-name = *ALPHA initial = ALPHA last-name = *ALPHA suffix = ("Jr." / "Sr." / 1*("I" / "V" / "X"))

street = [apt SP] house-num SP street-name CRLF apt = 1*4DIGIT house-num = 1*8(DIGIT / ALPHA) street-name = 1*VCHAR

zip-part = town-name "," SP state 1*2SP zip-code CRLF town-name = 1*(ALPHA / SP) state = 2ALPHA zip-code = 5DIGIT ["-" 4DIGIT]


RFC 5234 adds a warning in conjunction to the definition of LWSP as follows:

; 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.


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