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IETF RFC 2234
Augmented BNF for Syntax Specifications: ABNF
Last modified on Monday, November 17th, 1997
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Network Working Group D. Crocker, Ed.
Request for Comments: 2234 Internet Mail Consortium
Category: Standards Track P. Overell
Demon Internet Ltd.
November 1997
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 (1997). All Rights Reserved.
TABLE OF CONTENTS
1. INTRODUCTION .................................................. 2
2. RULE DEFINITION ............................................... 2
2.1 RULE NAMING .................................................. 2
2.2 RULE FORM .................................................... 3
2.3 TERMINAL VALUES .............................................. 3
2.4 EXTERNAL ENCODINGS ........................................... 5
3. OPERATORS ..................................................... 5
3.1 CONCATENATION RULE1 RULE2 ............................. 5
3.2 ALTERNATIVES RULE1 / RULE2 ................................... 6
3.3 INCREMENTAL ALTERNATIVES RULE1 =/ RULE2 .................... 6
3.4 VALUE RANGE ALTERNATIVES %C##-## ........................... 7
3.5 SEQUENCE GROUP (RULE1 RULE2) ................................. 7
3.6 VARIABLE REPETITION *RULE .................................... 8
3.7 SPECIFIC REPETITION NRULE .................................... 8
3.8 OPTIONAL SEQUENCE [RULE] ..................................... 8
3.9 ; COMMENT .................................................... 8
3.10 OPERATOR PRECEDENCE ......................................... 9
4. ABNF DEFINITION OF ABNF ....................................... 9
5. SECURITY CONSIDERATIONS ....................................... 10
Crocker & Overell Standards Track PAGE 1 
RFC 2234 ABNF for Syntax Specifications November 1997
6. APPENDIX A - CORE ............................................. 11
6.1 CORE RULES ................................................... 11
6.2 COMMON ENCODING .............................................. 12
7. ACKNOWLEDGMENTS ............................................... 12
8. REFERENCES .................................................... 13
9. CONTACT ....................................................... 13
10. FULL COPYRIGHT STATEMENT ..................................... 14
1. INTRODUCTION
Internet technical specifications often need to define a format
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
have come to be the common citations for defining ABNF. The current
document separates out that definition, 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 A (Core) 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 language defined in the body of this document,
and separate from its formal status.
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.
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RFC 2234 ABNF for Syntax Specifications November 1997
Unlike original BNF, angle brackets ("<", ">") are not required.
However, angle brackets may be used around a rule name whenever their
presence will facilitate 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.
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
Crocker & Overell Standards Track PAGE 3
RFC 2234 ABNF for Syntax Specifications November 1997
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 separation of characters within that value.
Hence:
CRLF = %d13.10
ABNF permits specifying literal text string directly, enclosed in
quotation-marks. Hence:
command = "command string"
Literal text strings are interpreted as a concatenated set of
printable characters.
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 which IS case SENSITIVE,
specify the characters individually.
For example:
rulename = %d97 %d98 %d99
or
rulename = %d97.98.99
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RFC 2234 ABNF for Syntax Specifications November 1997
will match only the string which comprises only 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.
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 freelyPand
implicitlyPinterspersed 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 which 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
Crocker & Overell Standards Track PAGE 5
RFC 2234 ABNF for Syntax Specifications November 1997
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 forward slash ("/") are alternatives.
Therefore,
foo / bar
will accept <foo> or <bar>.
NOTE: A quoted string containing alphabetic
characters is 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 which 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
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RFC 2234 ABNF for Syntax Specifications November 1997
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 can not 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
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
which matches (elem foo blat) or (elem bar blat).
elem foo / bar blat
matches (elem foo) or (bar blat).
NOTE: It is strongly advised to use grouping
notation, rather than to rely on proper reading of
"bare" alternations, when alternatives consist of
multiple rule names or literals.
Hence it is recommended that instead of the above form, the form:
(elem foo) / (bar blat)
be used. 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.
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RFC 2234 ABNF for Syntax Specifications November 1997
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 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.
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
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.
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RFC 2234 ABNF for Syntax Specifications November 1997
3.10 Operator Precedence
The various mechanisms described above have the following precedence,
from highest (binding tightest) at the top, to lowest and loosest at
the bottom:
Strings, Names formation
Comment
Value range
Repetition
Grouping, Optional
Concatenation
Alternative
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
This syntax uses the rules provided in Appendix A (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)
Crocker & Overell Standards Track PAGE 9
RFC 2234 ABNF for Syntax Specifications November 1997
concatenation = repetition *(1*c-wsp repetition)
repetition = [repeat] element
repeat = 1*DIGIT / (*DIGIT "*" *DIGIT)
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.
Crocker & Overell Standards Track PAGE 10
RFC 2234 ABNF for Syntax Specifications November 1997
6. APPENDIX A - CORE
This Appendix is provided as a convenient core for specific grammars.
The definitions may be used as a core set of rules.
6.1 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
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
Crocker & Overell Standards Track PAGE 11
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; space
VCHAR = %x21-7E
; visible (printing) characters
WSP = SP / HTAB
; white space
6.2 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" with the
higher-valued bytes represented on the left-hand side and being sent
over the network first.
7. ACKNOWLEDGMENTS
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 which 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 available over the last 15 years and therefore
to pursue enhancement. This turned the project into something rather
more ambitious than first intended. Interestingly the result is not
massively different from that original, although decisions such as
removing the list notation came as a surprise.
The current round of 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.
Crocker & Overell Standards Track PAGE 12
RFC 2234 ABNF for Syntax Specifications November 1997
8. REFERENCES
[US-ASCII] Coded Character Set--7-Bit American Standard Code for
Information Interchange, ANSI X3.4-1986.
[RFC 733] Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
"Standard for the Format of ARPA Network Text Message," RFC 733,
November 1977.
[RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, August 1982.
9. CONTACT
David H. Crocker Paul Overell
Internet Mail Consortium Demon Internet Ltd
675 Spruce Dr. Dorking Business Park
Sunnyvale, CA 94086 USA Dorking
Surrey, RH4 1HN
UK
Phone: +1 408 246 8253
Fax: +1 408 249 6205
EMail: dcrocker@imc.org paulo@turnpike.com
Crocker & Overell Standards Track PAGE 13
RFC 2234 ABNF for Syntax Specifications November 1997
10. Full Copyright Statement
Copyright © The Internet Society (1997). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Crocker & Overell Standards Track PAGE 14
Augmented BNF for Syntax Specifications: ABNF
RFC TOTAL SIZE: 24265 bytes
PUBLICATION DATE: Monday, November 17th, 1997
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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