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The augmented Backus–Naur form (ABNF) extends the Backus-Naur form.

The augmented Backus–Naur form (ABNF) is based on Backus–Naur form (BNF), but consists of its own syntax and derivation rules. The motive principle for this metalanguage is to describe a formal system of a language which is a protocol (bidirectional specification). It is documented in RFC 4234 and often serves as the definition language for IETF communication protocol.

RFC 4234 corrects problems in and obsoletes 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 7-bit ASCII in an 8-bit field with the high bit set to zero.

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.

Whitespace 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 or the rule the following rule could be constructed:

1. fubar = fu / bar

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

%c##-##

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

(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)

n*nRule

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

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]