RFC 4234
Augmented BNF for Syntax Specifications: ABNF
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 (C) 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.
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 ..........................................141. 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, [RFC733] and then [RFC822], 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
language defined in the body of this document, and separate from its formal status. Changes since [RFC2234]: 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.
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.
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.
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>.
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
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.
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
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)
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
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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
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.
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
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