[cite web]
url= http://www.rfc-editor.org/rfc-index2.html
title= "RFC Index"
accessdate= 2008-05-26
date= 2008-05-25
publisher= RFC Editor] Introduction
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:
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.
Operators
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.
Concatenation
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:
#fu = %x61 ; a
#bar = %x62 ; b
#mumble = fu bar fu
Alternation
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:
#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
#ruleset = alt1 / alt2 / alt3 / alt4 / alt5is equivalent to
#ruleset = alt1 / alt2
#ruleset =/ alt3
#ruleset =/ alt4 / alt5
Value range
%c##-##
A range of numeric values may be specified through the use of a hyphen (“-”).
The rule
#OCTAL = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"is equivalent to
#OCTAL = %x30-37
equence 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:
#group = elem (fubar / tarfu) snafu
To match “elem fubar” or “tarfu snafu” the following rules could be constructed:
#group = elem fubar / tarfu snafu
#group = (elem fubar) / (tarfu snafu)
Variable repetition
n*nRule
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.
pecific repetition
nRule
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
[Rule]
To indicate an optional element the following constructions are equivalent:
# [fubar snafu]
#*1(fubar snafu)
#0*1(fubar snafu)
Comment
; 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:
#Strings, Names formation
#Comment
#Value range
#Repetition
#Grouping, Optional
#Concatenation
#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.
Example
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] CRLFname-part =/ personal-part CRLF
personal-part = first-name / (initial ".")first-name = *ALPHAinitial = ALPHAlast-name = *ALPHAsuffix = ("Jr." / "Sr." / 1*("I" / "V" / "X"))
street = [apt SP] house-num SP street-name CRLFapt = 1*4DIGIThouse-num = 1*8(DIGIT / ALPHA)street-name = 1*VCHAR
zip-part = town-name "," SP state 1*2SP zip-code CRLFtown-name = 1*(ALPHA / SP)state = 2ALPHAzip-code = 5DIGIT ["-" 4DIGIT]
Pitfalls
[http://www.ietf.org/rfc/rfc5234.txt 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.
References
