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rfc:rfc2234

Network Working Group D. Crocker, Ed. Request for Comments: 2234 Internet Mail Consortium Category: Standards Track P. Overell

                                                    Demon Internet Ltd.
                                                          November 1997
           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 (1997).  All Rights Reserved.

TABLE OF CONTENTS

 1. INTRODUCTION ..................................................  2
 2. RULE DEFINITION ...............................................  2
 2.1 RULE NAMING ..................................................  2
 2.2 RULE FORM ....................................................  3
 2.3 TERMINAL VALUES ..............................................  3
 2.4 EXTERNAL ENCODINGS ...........................................  5
 3. OPERATORS .....................................................  5
 3.1 CONCATENATION    RULE1     RULE2 .............................  5
 3.2 ALTERNATIVES RULE1 / RULE2 ...................................  6
 3.3 INCREMENTAL ALTERNATIVES   RULE1 =/ RULE2 ....................  6
 3.4 VALUE RANGE ALTERNATIVES   %C##-## ...........................  7
 3.5 SEQUENCE GROUP (RULE1 RULE2) .................................  7
 3.6 VARIABLE REPETITION *RULE ....................................  8
 3.7 SPECIFIC REPETITION NRULE ....................................  8
 3.8 OPTIONAL SEQUENCE [RULE] .....................................  8
 3.9 ; COMMENT ....................................................  8
 3.10 OPERATOR PRECEDENCE .........................................  9
 4. ABNF DEFINITION OF ABNF .......................................  9
 5. SECURITY CONSIDERATIONS ....................................... 10

Crocker & Overell Standards Track [Page 1] RFC 2234 ABNF for Syntax Specifications November 1997

 6. APPENDIX A - CORE ............................................. 11
 6.1 CORE RULES ................................................... 11
 6.2 COMMON ENCODING .............................................. 12
 7. ACKNOWLEDGMENTS ............................................... 12
 8. REFERENCES .................................................... 13
 9. CONTACT ....................................................... 13
 10. FULL COPYRIGHT STATEMENT ..................................... 14

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
 have come 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 A (Core) 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.

Crocker & Overell Standards Track [Page 2] RFC 2234 ABNF for Syntax Specifications November 1997

 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

Crocker & Overell Standards Track [Page 3] RFC 2234 ABNF for Syntax Specifications November 1997

 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

Crocker & Overell Standards Track [Page 4] RFC 2234 ABNF for Syntax Specifications November 1997

 will match only the string which comprises only 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 freelyPand
      implicitlyPinterspersed 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 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

Crocker & Overell Standards Track [Page 5] RFC 2234 ABNF for Syntax Specifications November 1997

 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 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

Crocker & Overell Standards Track [Page 6] RFC 2234 ABNF for Syntax Specifications November 1997

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.

Crocker & Overell Standards Track [Page 7] RFC 2234 ABNF for Syntax Specifications November 1997

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

 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.

Crocker & Overell Standards Track [Page 8] RFC 2234 ABNF for Syntax Specifications November 1997

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:
      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

 This syntax uses the rules provided in Appendix A (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)

Crocker & Overell Standards Track [Page 9] RFC 2234 ABNF for Syntax Specifications November 1997

      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.

Crocker & Overell Standards Track [Page 10] RFC 2234 ABNF for Syntax Specifications November 1997

6. APPENDIX A - CORE

 This Appendix is provided as a convenient core for specific grammars.
 The definitions may be used as a core set of rules.

6.1 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

Crocker & Overell Standards Track [Page 11] RFC 2234 ABNF for Syntax Specifications November 1997

                             ; space
      VCHAR          =  %x21-7E
                             ; visible (printing) characters
      WSP            =  SP / HTAB
                             ; white space

6.2 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.

7. ACKNOWLEDGMENTS

 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 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.
 The current round of 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.

Crocker & Overell Standards Track [Page 12] RFC 2234 ABNF for Syntax Specifications November 1997

8. REFERENCES

 [US-ASCII]     Coded Character Set--7-Bit American Standard Code for
 Information Interchange, ANSI X3.4-1986.
 [RFC733]  Crocker, D., Vittal, J., Pogran, K., and D. Henderson,
 "Standard for the Format of ARPA Network Text Message," RFC 733,
 November 1977.
 [RFC822]  Crocker, D., "Standard for the Format of ARPA Internet Text
 Messages", STD 11, RFC 822, August 1982.

9. CONTACT

 David H. Crocker                 Paul Overell
 Internet Mail Consortium         Demon Internet Ltd
 675 Spruce Dr.                   Dorking Business Park
 Sunnyvale, CA 94086 USA          Dorking
                                  Surrey, RH4 1HN
                                  UK
 Phone:    +1 408 246 8253
 Fax:      +1 408 249 6205
 EMail:    dcrocker@imc.org       paulo@turnpike.com

Crocker & Overell Standards Track [Page 13] RFC 2234 ABNF for Syntax Specifications November 1997

10. Full Copyright Statement

 Copyright (C) The Internet Society (1997).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Crocker & Overell Standards Track [Page 14]

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