











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









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   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.









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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











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   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







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   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

















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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



   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.

















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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)









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        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.   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







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                               ; 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.























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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





















































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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.

















































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