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RFC 5234 - Augmented BNF for Syntax Specifications: ABNF


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Network Working Group                                    D. Crocker, Ed.
Request for Comments: 5234                   Brandenburg InternetWorking
STD: 68                                                       P. Overell
Obsoletes: 4234                                                THUS plc.
Category: Standards Track                                   January 2008

             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.

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 . . . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . . . . 10
   4.  ABNF Definition of ABNF  . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13
   Appendix B.  Core ABNF of ABNF . . . . . . . . . . . . . . . . . . 13
     B.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . . . . 13
     B.2.  Common Encoding  . . . . . . . . . . . . . . . . . . . . . 15

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, [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.

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 lowercase
   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 B 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 lowercase.

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 ruleset, such as
   often occurs with parameter lists.  ABNF permits this incremental
   definition through the construct:

         oldrule     =/ additional-alternatives

   So that the ruleset

         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 a 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 semicolon 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:

      Rule name, prose-val, Terminal value

      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.

         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

   [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.

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.  Core ABNF of ABNF

   This appendix contains some basic rules that are in common use.
   Basic rules are in uppercase.  Note that these rules are only valid
   for ABNF encoded in 7-bit ASCII or in characters sets that are a
   superset of 7-bit ASCII.

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)
                                ; 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.

         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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