rfc5234.txt

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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.






















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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























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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.







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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.







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   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.








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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 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.



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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











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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.





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