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IETF RFC 4234
Augmented BNF for Syntax Specifications: ABNF
Last modified on Monday, October 3rd, 2005
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Network Working Group D. Crocker, Ed.
Request for Comments: 4234 Brandenburg InternetWorking
Obsoletes: 2234 P. Overell
Category: Standards Track THUS plc.
October 2005
Augmented BNF for Syntax Specifications: ABNF
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2005).
Abstract
Internet technical specifications often need to define a formal
syntax. Over the years, a modified version of Backus-Naur Form
(BNF), called Augmented BNF (ABNF), has been popular among many
Internet specifications. The current specification documents ABNF.
It balances compactness and simplicity, with reasonable
representational power. The differences between standard BNF and
ABNF involve naming rules, repetition, alternatives, order-
independence, and value ranges. This specification also supplies
additional rule definitions and encoding for a core lexical analyzer
of the type common to several Internet specifications.
Crocker & Overell Standards Track PAGE 1 
RFC 4234 ABNF October 2005
Table of Contents
1. INTRODUCTION ....................................................2
2. RULE DEFINITION .................................................3
2.1. Rule Naming ................................................3
2.2. Rule Form ..................................................3
2.3. Terminal Values ............................................4
2.4. External Encodings .........................................5
3. OPERATORS .......................................................6
3.1. Concatenation: Rule1 Rule2 ................................6
3.2. Alternatives: Rule1 / Rule2 ...............................6
3.3. Incremental Alternatives: Rule1 =/ Rule2 ...................7
3.4. Value Range Alternatives: %c##-## .........................7
3.5. Sequence Group: (Rule1 Rule2) .............................8
3.6. Variable Repetition: *Rule ................................8
3.7. Specific Repetition: nRule ................................9
3.8. Optional Sequence: [RULE] .................................9
3.9. Comment: ; Comment ........................................9
3.10. Operator Precedence .......................................9
4. ABNF DEFINITION OF ABNF ........................................10
5. SECURITY CONSIDERATIONS ........................................11
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................11
Appendix A. ACKNOWLEDGEMENTS .....................................13
Appendix B. APPENDIX - CORE ABNF OF ABNF .........................13
B.1. Core Rules ...............................................13
B.2. Common Encoding ..........................................14
1. INTRODUCTION
Internet technical specifications often need to define a formal
syntax and are free to employ whatever notation their authors deem
useful. Over the years, a modified version of Backus-Naur Form
(BNF), called Augmented BNF (ABNF), has been popular among many
Internet specifications. It balances compactness and simplicity,
with reasonable representational power. In the early days of the
Arpanet, each specification contained its own definition of ABNF.
This included the email specifications, [RFC 733] and then [RFC 822],
which came to be the common citations for defining ABNF. The current
document separates those definitions to permit selective reference.
Predictably, it also provides some modifications and enhancements.
The differences between standard BNF and ABNF involve naming rules,
repetition, alternatives, order-independence, and value ranges.
Appendix B supplies rule definitions and encoding for a core lexical
analyzer of the type common to several Internet specifications. It
is provided as a convenience and is otherwise separate from the meta
Crocker & Overell Standards Track PAGE 2
RFC 4234 ABNF October 2005
language defined in the body of this document, and separate from its
formal status.
Changes since [RFC 2234]:
In Section 3.7, the phrase: "That is, exactly <N> occurrences of
<element>." was corrected to: "That is, exactly <n> occurrences of
<element>."
Some continuation comment lines needed to be corrected to begin
with comment character (";").
2. RULE DEFINITION
2.1. Rule Naming
The name of a rule is simply the name itself; that is, a sequence of
characters, beginning with an alphabetic character, and followed by a
combination of alphabetics, digits, and hyphens (dashes).
NOTE:
Rule names are case-insensitive
The names <rulename>, <Rulename>, <RULENAME>, and <rUlENamE> all
refer to the same rule.
Unlike original BNF, angle brackets ("<", ">") are not required.
However, angle brackets may be used around a rule name whenever their
presence facilitates in discerning the use of a rule name. This is
typically restricted to rule name references in free-form prose, or
to distinguish partial rules that combine into a string not separated
by white space, such as shown in the discussion about repetition,
below.
2.2. Rule Form
A rule is defined by the following sequence:
name = elements crlf
where <name> is the name of the rule, <elements> is one or more rule
names or terminal specifications, and <crlf> is the end-of-line
indicator (carriage return followed by line feed). The equal sign
separates the name from the definition of the rule. The elements
form a sequence of one or more rule names and/or value definitions,
combined according to the various operators defined in this document,
such as alternative and repetition.
Crocker & Overell Standards Track PAGE 3
RFC 4234 ABNF October 2005
For visual ease, rule definitions are left aligned. When a rule
requires multiple lines, the continuation lines are indented. The
left alignment and indentation are relative to the first lines of the
ABNF rules and need not match the left margin of the document.
2.3. Terminal Values
Rules resolve into a string of terminal values, sometimes called
characters. In ABNF, a character is merely a non-negative integer.
In certain contexts, a specific mapping (encoding) of values into a
character set (such as ASCII) will be specified.
Terminals are specified by one or more numeric characters, with the
base interpretation of those characters indicated explicitly. The
following bases are currently defined:
b = binary
d = decimal
x = hexadecimal
Hence:
CR = %d13
CR = %x0D
respectively specify the decimal and hexadecimal representation of
[US-ASCII] for carriage return.
A concatenated string of such values is specified compactly, using a
period (".") to indicate a separation of characters within that
value. Hence:
CRLF = %d13.10
ABNF permits the specification of literal text strings directly,
enclosed in quotation-marks. Hence:
command = "command string"
Literal text strings are interpreted as a concatenated set of
printable characters.
Crocker & Overell Standards Track PAGE 4
RFC 4234 ABNF October 2005
NOTE:
ABNF strings are case-insensitive and the character set for these
strings is us-ascii.
Hence:
rulename = "abc"
and:
rulename = "aBc"
will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and
"ABC".
To specify a rule that IS case SENSITIVE, specify the characters
individually.
For example:
rulename = %d97 %d98 %d99
or
rulename = %d97.98.99
will match only the string that comprises only the lowercased
characters, abc.
2.4. External Encodings
External representations of terminal value characters will vary
according to constraints in the storage or transmission environment.
Hence, the same ABNF-based grammar may have multiple external
encodings, such as one for a 7-bit US-ASCII environment, another for
a binary octet environment, and still a different one when 16-bit
Unicode is used. Encoding details are beyond the scope of ABNF,
although Appendix A (Core) provides definitions for a 7-bit US-ASCII
environment as has been common to much of the Internet.
By separating external encoding from the syntax, it is intended that
alternate encoding environments can be used for the same syntax.
Crocker & Overell Standards Track PAGE 5
RFC 4234 ABNF October 2005
3. OPERATORS
3.1. Concatenation: Rule1 Rule2
A rule can define a simple, ordered string of values (i.e., a
concatenation of contiguous characters) by listing a sequence of rule
names. For example:
foo = %x61 ; a
bar = %x62 ; b
mumble = foo bar foo
So that the rule <mumble> matches the lowercase string "aba".
LINEAR WHITE SPACE: Concatenation is at the core of the ABNF parsing
model. A string of contiguous characters (values) is parsed
according to the rules defined in ABNF. For Internet specifications,
there is some history of permitting linear white space (space and
horizontal tab) to be freely and implicitly interspersed around major
constructs, such as delimiting special characters or atomic strings.
NOTE:
This specification for ABNF does not provide for implicit
specification of linear white space.
Any grammar that wishes to permit linear white space around
delimiters or string segments must specify it explicitly. It is
often useful to provide for such white space in "core" rules that are
then used variously among higher-level rules. The "core" rules might
be formed into a lexical analyzer or simply be part of the main
ruleset.
3.2. Alternatives: Rule1 / Rule2
Elements separated by a forward slash ("/") are alternatives.
Therefore,
foo / bar
will accept <foo> or <bar>.
Crocker & Overell Standards Track PAGE 6
RFC 4234 ABNF October 2005
NOTE:
A quoted string containing alphabetic characters is a special form
for specifying alternative characters and is interpreted as a
non-terminal representing the set of combinatorial strings with
the contained characters, in the specified order but with any
mixture of upper and lower case.
3.3. Incremental Alternatives: Rule1 =/ Rule2
It is sometimes convenient to specify a list of alternatives in
fragments. That is, an initial rule may match one or more
alternatives, with later rule definitions adding to the set of
alternatives. This is particularly useful for otherwise, independent
specifications that derive from the same parent rule set, such as
often occurs with parameter lists. ABNF permits this incremental
definition through the construct:
oldrule =/ additional-alternatives
So that the rule set
ruleset = alt1 / alt2
ruleset =/ alt3
ruleset =/ alt4 / alt5
is the same as specifying
ruleset = alt1 / alt2 / alt3 / alt4 / alt5
3.4. Value Range Alternatives: %c##-##
A range of alternative numeric values can be specified compactly,
using dash ("-") to indicate the range of alternative values. Hence:
DIGIT = %x30-39
is equivalent to:
DIGIT = "0" / "1" / "2" / "3" / "4" / "5" / "6" /
"7" / "8" / "9"
Concatenated numeric values and numeric value ranges cannot be
specified in the same string. A numeric value may use the dotted
notation for concatenation or it may use the dash notation to specify
Crocker & Overell Standards Track PAGE 7
RFC 4234 ABNF October 2005
one value range. Hence, to specify one printable character between
end of line sequences, the specification could be:
char-line = %x0D.0A %x20-7E %x0D.0A
3.5. Sequence Group: (Rule1 Rule2)
Elements enclosed in parentheses are treated as a single element,
whose contents are STRICTLY ORDERED. Thus,
elem (foo / bar) blat
matches (elem foo blat) or (elem bar blat), and
elem foo / bar blat
matches (elem foo) or (bar blat).
NOTE:
It is strongly advised that grouping notation be used, rather than
relying on the proper reading of "bare" alternations, when
alternatives consist of multiple rule names or literals.
Hence, it is recommended that the following form be used:
(elem foo) / (bar blat)
It will avoid misinterpretation by casual readers.
The sequence group notation is also used within free text to set off
an element sequence from the prose.
3.6. Variable Repetition: *Rule
The operator "*" preceding an element indicates repetition. The full
form is:
<a>*<b>element
where <a> and <b> are optional decimal values, indicating at least
<a> and at most <b> occurrences of the element.
Default values are 0 and infinity so that *<element> allows any
number, including zero; 1*<element> requires at least one;
3*3<element> allows exactly 3 and 1*2<element> allows one or two.
Crocker & Overell Standards Track PAGE 8
RFC 4234 ABNF October 2005
3.7. Specific Repetition: nRule
A rule of the form:
<n>element
is equivalent to
<n>*<n>element
That is, exactly <n> occurrences of <element>. Thus, 2DIGIT is a 2-
digit number, and 3ALPHA is a string of three alphabetic characters.
3.8. Optional Sequence: [RULE]
Square brackets enclose an optional element sequence:
[foo bar]
is equivalent to
*1(foo bar).
3.9. Comment: ; Comment
A semi-colon starts a comment that continues to the end of line.
This is a simple way of including useful notes in parallel with the
specifications.
3.10. Operator Precedence
The various mechanisms described above have the following precedence,
from highest (binding tightest) at the top, to lowest (loosest) at
the bottom:
Strings, Names formation
Comment
Value range
Repetition
Grouping, Optional
Concatenation
Alternative
Crocker & Overell Standards Track PAGE 9
RFC 4234 ABNF October 2005
Use of the alternative operator, freely mixed with concatenations,
can be confusing.
Again, it is recommended that the grouping operator be used to
make explicit concatenation groups.
4. ABNF DEFINITION OF ABNF
NOTES:
1. This syntax requires a formatting of rules that is relatively
strict. Hence, the version of a ruleset included in a
specification might need preprocessing to ensure that it can be
interpreted by an ABNF parser.
2. This syntax uses the rules provided in Appendix B (Core).
rulelist = 1*( rule / (*c-wsp c-nl) )
rule = rulename defined-as elements c-nl
; continues if next line starts
; with white space
rulename = ALPHA *(ALPHA / DIGIT / "-")
defined-as = *c-wsp ("=" / "=/") *c-wsp
; basic rules definition and
; incremental alternatives
elements = alternation *c-wsp
c-wsp = WSP / (c-nl WSP)
c-nl = comment / CRLF
; comment or newline
comment = ";" *(WSP / VCHAR) CRLF
alternation = concatenation
*(*c-wsp "/" *c-wsp concatenation)
concatenation = repetition *(1*c-wsp repetition)
repetition = [repeat] element
repeat = 1*DIGIT / (*DIGIT "*" *DIGIT)
Crocker & Overell Standards Track PAGE 10
RFC 4234 ABNF October 2005
element = rulename / group / option /
char-val / num-val / prose-val
group = "(" *c-wsp alternation *c-wsp ")"
option = "[" *c-wsp alternation *c-wsp "]"
char-val = DQUOTE *(%x20-21 / %x23-7E) DQUOTE
; quoted string of SP and VCHAR
; without DQUOTE
num-val = "%" (bin-val / dec-val / hex-val)
bin-val = "b" 1*BIT
[ 1*("." 1*BIT) / ("-" 1*BIT) ]
; series of concatenated bit values
; or single ONEOF range
dec-val = "d" 1*DIGIT
[ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]
hex-val = "x" 1*HEXDIG
[ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ]
prose-val = "<" *(%x20-3D / %x3F-7E) ">"
; bracketed string of SP and VCHAR
; without angles
; prose description, to be used as
; last resort
5. SECURITY CONSIDERATIONS
Security is truly believed to be irrelevant to this document.
6. References
6.1. Normative References
[US-ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
6.2. Informative References
[RFC 2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
Crocker & Overell Standards Track PAGE 11
RFC 4234 ABNF October 2005
[RFC 733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the format of ARPA network text messages",
RFC 733, November 1977.
[RFC 822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
Crocker & Overell Standards Track PAGE 12
RFC 4234 ABNF October 2005
Appendix A. ACKNOWLEDGEMENTS
The syntax for ABNF was originally specified in RFC 733. Ken L.
Harrenstien, of SRI International, was responsible for re-coding the
BNF into an augmented BNF that makes the representation smaller and
easier to understand.
This recent project began as a simple effort to cull out the portion
of RFC 822 that has been repeatedly cited by non-email specification
writers, namely the description of augmented BNF. Rather than simply
and blindly converting the existing text into a separate document,
the working group chose to give careful consideration to the
deficiencies, as well as benefits, of the existing specification and
related specifications made available over the last 15 years, and
therefore to pursue enhancement. This turned the project into
something rather more ambitious than was first intended.
Interestingly, the result is not massively different from that
original, although decisions, such as removing the list notation,
came as a surprise.
This "separated" version of the specification was part of the DRUMS
working group, with significant contributions from Jerome Abela,
Harald Alvestrand, Robert Elz, Roger Fajman, Aviva Garrett, Tom
Harsch, Dan Kohn, Bill McQuillan, Keith Moore, Chris Newman, Pete
Resnick, and Henning Schulzrinne.
Julian Reschke warrants a special thanks for converting the Draft
Standard version to XML source form.
Appendix B. APPENDIX - CORE ABNF OF ABNF
This Appendix is provided as a convenient core for specific grammars.
The definitions may be used as a core set of rules.
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
Crocker & Overell Standards Track PAGE 13
RFC 4234 ABNF October 2005
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)
; linear white space (past newline)
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.
Crocker & Overell Standards Track PAGE 14
RFC 4234 ABNF October 2005
Authors' Addresses
Dave Crocker (editor)
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
US
Phone: +1.408.246.8253
EMail: dcrocker@bbiw.net
Paul Overell
THUS plc.
1/2 Berkeley Square
99 Berkeley Street
Glasgow
G3 7HR
UK
EMail: paul.overell@thus.net
Crocker & Overell Standards Track PAGE 15
RFC 4234 ABNF October 2005
Full Copyright Statement
Copyright © The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on an
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Crocker & Overell Standards Track PAGE 16
Augmented BNF for Syntax Specifications: ABNF
RFC TOTAL SIZE: 26351 bytes
PUBLICATION DATE: Monday, October 3rd, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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