Augmented BNF for Syntax Specifications: ABNF
draft-crocker-abnf-rfc2234bis-00

Network Working Group                                    D. Crocker, Ed.
Internet-Draft                               Brandenburg InternetWorking
Obsoletes: RFC2234 (if approved)                              P. Overell
Expires: September 10, 2005                          Demon Internet Ltd.
                                                           March 9, 2005

             Augmented BNF for Syntax Specifications: ABNF
                    draft-crocker-abnf-rfc2234bis-00

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Internet technical specifications often need to define a format
   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

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   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 . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   RULE DEFINITION  . . . . . . . . . . . . . . . . . . . . . .   3
   2.1  Rule Naming  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.2  Rule Form  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.3  Terminal Values  . . . . . . . . . . . . . . . . . . . . . .   4
   2.4  External Encodings . . . . . . . . . . . . . . . . . . . . .   6
   3.   OPERATORS  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.1  Concatenation:  Rule1 Rule2  . . . . . . . . . . . . . . . .   6
   3.2  Alternatives:  Rule1 / Rule2 . . . . . . . . . . . . . . . .   7
   3.3  Incremental Alternatives: Rule1 =/ Rule2 . . . . . . . . . .   7
   3.4  Value Range Alternatives:  %c##-## . . . . . . . . . . . . .   8
   3.5  Sequence Group:  (Rule1 Rule2) . . . . . . . . . . . . . . .   8
   3.6  Variable Repetition:  *Rule  . . . . . . . . . . . . . . . .   9
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.1  Normative  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.2  Descriptive  . . . . . . . . . . . . . . . . . . . . . . . .  12
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  12
   A.   ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . .  12
   B.   APPENDIX - CORE ABNF OF ABNF . . . . . . . . . . . . . . . .  13
   B.1  Core Rules . . . . . . . . . . . . . . . . . . . . . . . . .  13
   B.2  Common Encoding  . . . . . . . . . . . . . . . . . . . . . .  14
        Intellectual Property and Copyright Statements . . . . . . .  15

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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, [RFC733] and then [RFC822]
   which came 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 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 in the latest version of this Internet Draft:

      In Section 3.7 the phrase: "That is, exactly <N>Ã occurrences of
      <element>." was correct to: "That is, exactly <n>Ã 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.

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   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

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   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
   will match only the string which comprises only lowercased
   characters, abc.

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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:

      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
   then used variously among higher-level rules.  The "core" rules might
   be formed into a lexical analyzer or simply be part of the main

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   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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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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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:  ; 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 and loosest at
   the bottom:

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      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 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)

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           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.

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6.  References

6.1  Normative

   [US-ASCII]
              American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

6.2  Descriptive

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC733]   Crocker, D., Vittal, J., Pogran, K. and D. Henderson,
              "Standard for the format of ARPA network text messages",
              RFC 733, November 1977.

   [RFC822]   Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

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
   Demon Internet Ltd.
   Dorking Business Park
   Dorking
   Surrey, England  RH4 1HN
   UK

   Email: paulo@turnpike.com

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

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   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.

   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 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

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                                  ; 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" with the
   higher-valued bytes represented on the left-hand side and being sent
   over the network first.

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Intellectual Property Statement

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   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Disclaimer of Validity

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright Statement

   Copyright (C) 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 retain all their rights.

Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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