rfc5234.txt

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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.





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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 (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 / "-")






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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) ]






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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.

























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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



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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







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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: [phone]
   EMail: [e-mail]


   Paul Overell
   THUS plc.
   1/2 Berkeley Square,
   99 Berkeley Street
   Glasgow  G3 7HR
   UK

   EMail: [e-mail]























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RFC 5234                          ABNF                      January 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

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   contained in BCP 78, and except as set forth therein, the authors
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