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Network Working Group                                    D. Crocker, Ed.Request for Comments: 4234                   Brandenburg InternetWorkingObsoletes:2234                                               P. OverellCategory: Standards Track                                      THUS plc.                                                            October 2005Augmented BNF for Syntax Specifications: ABNFStatus 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 (2005).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 4234                          ABNF                      October 2005Table of Contents1. INTRODUCTION ....................................................22. RULE DEFINITION .................................................32.1. Rule Naming ................................................32.2. Rule Form ..................................................32.3. Terminal Values ............................................42.4. External Encodings .........................................53. OPERATORS .......................................................63.1. Concatenation:  Rule1 Rule2 ................................63.2. Alternatives:  Rule1 / Rule2 ...............................63.3. Incremental Alternatives: Rule1 =/ Rule2 ...................73.4. Value Range Alternatives:  %c##-## .........................73.5. Sequence Group:  (Rule1 Rule2) .............................83.6. Variable Repetition:  *Rule ................................83.7. Specific Repetition:  nRule ................................93.8. Optional Sequence:  [RULE] .................................93.9. Comment:  ; Comment ........................................93.10. Operator Precedence .......................................94. ABNF DEFINITION OF ABNF ........................................105. SECURITY CONSIDERATIONS ........................................116. References .....................................................116.1. Normative References ......................................116.2. Informative References ....................................11Appendix A.  ACKNOWLEDGEMENTS .....................................13Appendix B.  APPENDIX - CORE ABNF OF ABNF .........................13B.1.  Core Rules ...............................................13B.2.  Common Encoding ..........................................141.  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 metaCrocker & Overell           Standards Track                     [Page 2]

RFC 4234                          ABNF                      October 2005   language defined in the body of this document, and separate from its   formal status.   Changes since [RFC2234]:      InSection 3.7, the phrase: "That is, exactly <N> occurrences of      <element>." was corrected to: "That is, exactly <n> occurrences of      <element>."      Some continuation comment lines needed to be corrected to begin      with comment character (";").2.  RULE DEFINITION2.1.  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.Crocker & Overell           Standards Track                     [Page 3]

RFC 4234                          ABNF                      October 2005   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.Crocker & Overell           Standards Track                     [Page 4]

RFC 4234                          ABNF                      October 2005   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 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,   althoughAppendix 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.Crocker & Overell           Standards Track                     [Page 5]

RFC 4234                          ABNF                      October 20053.  OPERATORS3.1.  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>.Crocker & Overell           Standards Track                     [Page 6]

RFC 4234                          ABNF                      October 2005   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 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 that 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 / alt53.4.  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 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 specifyCrocker & Overell           Standards Track                     [Page 7]

RFC 4234                          ABNF                      October 2005   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.0A3.5.  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.Crocker & Overell           Standards Track                     [Page 8]

RFC 4234                          ABNF                      October 20053.7.  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 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.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:         Strings, Names formation         Comment         Value range         Repetition         Grouping, Optional         Concatenation         AlternativeCrocker & Overell           Standards Track                     [Page 9]

RFC 4234                          ABNF                      October 2005   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 inAppendix B (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)         concatenation  =  repetition *(1*c-wsp repetition)         repetition     =  [repeat] element         repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)Crocker & Overell           Standards Track                    [Page 10]

RFC 4234                          ABNF                      October 2005         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 resort5.  SECURITY CONSIDERATIONS   Security is truly believed to be irrelevant to this document.6.  References6.1.  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   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF",RFC 2234, November 1997.Crocker & Overell           Standards Track                    [Page 11]

RFC 4234                          ABNF                      October 2005   [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 4234                          ABNF                      October 2005Appendix A.  ACKNOWLEDGEMENTS   The syntax for ABNF was originally specified inRFC 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   ofRFC 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.  APPENDIX - CORE ABNF OF ABNF   This Appendix is provided as a convenient core for specific grammars.   The definitions may be used as a core set of rules.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 NULCrocker & Overell           Standards Track                    [Page 13]

RFC 4234                          ABNF                      October 2005         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         VCHAR          =  %x21-7E                                ; visible (printing) characters         WSP            =  SP / HTAB                                ; white spaceB.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.Crocker & Overell           Standards Track                    [Page 14]

RFC 4234                          ABNF                      October 2005Authors' 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.netCrocker & Overell           Standards Track                    [Page 15]

RFC 4234                          ABNF                      October 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Crocker & Overell           Standards Track                    [Page 16]