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rfc:rfc5234

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.

Crocker & Overell Standards Track [Page 1] RFC 5234 ABNF January 2008

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

Crocker & Overell Standards Track [Page 2] RFC 5234 ABNF January 2008

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.

Crocker & Overell Standards Track [Page 3] RFC 5234 ABNF January 2008

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.

Crocker & Overell Standards Track [Page 4] RFC 5234 ABNF January 2008

 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.

Crocker & Overell Standards Track [Page 5] RFC 5234 ABNF January 2008

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.

Crocker & Overell Standards Track [Page 6] RFC 5234 ABNF January 2008

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

Crocker & Overell Standards Track [Page 7] RFC 5234 ABNF January 2008

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.

Crocker & Overell Standards Track [Page 8] RFC 5234 ABNF January 2008

 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.

Crocker & Overell Standards Track [Page 9] RFC 5234 ABNF January 2008

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 / "-")

Crocker & Overell Standards Track [Page 10] RFC 5234 ABNF January 2008

       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) ]

Crocker & Overell Standards Track [Page 11] RFC 5234 ABNF January 2008

       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.

Crocker & Overell Standards Track [Page 12] RFC 5234 ABNF January 2008

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

Crocker & Overell Standards Track [Page 13] RFC 5234 ABNF January 2008

       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

Crocker & Overell Standards Track [Page 14] RFC 5234 ABNF January 2008

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

Crocker & Overell Standards Track [Page 15] RFC 5234 ABNF January 2008

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Crocker & Overell Standards Track [Page 16]

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