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264       content: "RFC 5234";
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267       content: "January 2008";
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270       content: "ABNF";
271  }
272  @bottom-left {
273       content: "Crocker & Overell";
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294</style><link rel="Contents" href="#rfc.toc">
295      <link rel="Author" href="#rfc.authors">
296      <link rel="Copyright" href="#rfc.copyright">
297      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
298      <link rel="Chapter" title="2 Rule Definition" href="#rfc.section.2">
299      <link rel="Chapter" title="3 Operators" href="#rfc.section.3">
300      <link rel="Chapter" title="4 ABNF Definition of ABNF" href="#rfc.section.4">
301      <link rel="Chapter" title="5 Security Considerations" href="#rfc.section.5">
302      <link rel="Chapter" href="#rfc.section.6" title="6 References">
303      <link rel="Appendix" title="A Acknowledgements" href="#rfc.section.A">
304      <link rel="Appendix" title="B Core ABNF of ABNF" href="#rfc.section.B">
305      <link rel="Alternate" title="Authorative ASCII Version" href="http://www.ietf.org/rfc/rfc5234.txt">
306      <link rel="Help" title="Additional Information on tools.ietf.org" href="http://tools.ietf.org/html/rfc5234">
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308      <meta name="keywords" content="ABNF, Augmented, Backus-Naur, Form, electronic, mail">
309      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
310      <meta name="DC.Creator" content="Crocker, D.">
311      <meta name="DC.Creator" content="Overell, P.">
312      <meta name="DC.Identifier" content="urn:ietf:rfc:5234">
313      <meta name="DC.Date.Issued" scheme="ISO8601" content="2008-01">
314      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:4234">
315      <meta name="DC.Description.Abstract" content="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.">
316      <meta name="DC.isPartOf" content="urn:ISSN:2070-1721">
317   </head>
318   <body>
319      <!--WARNING: document uses ABNF syntax, but doesn't reference RFC 2234, 4234 or 5234.-->
320      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
321         <tr>
322            <td class="header left">Network Working Group</td>
323            <td class="header right">D. Crocker, Editor</td>
324         </tr>
325         <tr>
326            <td class="header left">Request for Comments: 5234</td>
327            <td class="header right">Brandenburg InternetWorking</td>
328         </tr>
329         <tr>
330            <td class="header left">Obsoletes:
331               <!--WARNING: front matter mentions RFC 4234 for which there is no reference element--><a href="http://tools.ietf.org/html/rfc4234">4234</a></td>
332            <td class="header right">P. Overell</td>
333         </tr>
334         <tr>
335            <td class="header left">STD: 68</td>
336            <td class="header right">THUS plc.</td>
337         </tr>
338         <tr>
339            <td class="header left">Category: Standards Track</td>
340            <td class="header right">January 2008</td>
341         </tr>
342      </table>
343      <p class="title">Augmented BNF for Syntax Specifications: ABNF</p>
344      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
345      <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
346         for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
347         state and status of this protocol. Distribution of this memo is unlimited.
348      </p>
349      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
350      <p>Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur
351         Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents
352         ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF
353         and ABNF involve naming rules, repetition, alternatives, order- independence, and value ranges. This specification also supplies
354         additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications.
355      </p>
356      <hr class="noprint">
357      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
358      <ul class="toc">
359         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Introduction</a></li>
360         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Rule Definition</a><ul class="toc">
361               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.1">Rule Naming</a></li>
362               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.2">Rule Form</a></li>
363               <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.3">Terminal Values</a></li>
364               <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.4">External Encodings</a></li>
365            </ul>
366         </li>
367         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Operators</a><ul class="toc">
368               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Concatenation:  Rule1 Rule2</a></li>
369               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#Alternatives">Alternatives:  Rule1 / Rule2</a></li>
370               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#Incremental">Incremental Alternatives: Rule1 =/ Rule2</a></li>
371               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#Range">Value Range Alternatives:  %c##-##</a></li>
372               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#Sequence">Sequence Group:  (Rule1 Rule2)</a></li>
373               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#VarRep">Variable Repetition:  *Rule</a></li>
374               <li class="tocline1">3.7&nbsp;&nbsp;&nbsp;<a href="#SpecRep">Specific Repetition:  nRule</a></li>
375               <li class="tocline1">3.8&nbsp;&nbsp;&nbsp;<a href="#OptSeq">Optional Sequence:  [RULE]</a></li>
376               <li class="tocline1">3.9&nbsp;&nbsp;&nbsp;<a href="#Comment">Comment:  ; Comment</a></li>
377               <li class="tocline1">3.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.10">Operator Precedence</a></li>
378            </ul>
379         </li>
380         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4">ABNF Definition of ABNF</a></li>
381         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Security Considerations</a></li>
382         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
383               <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
384               <li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
385            </ul>
386         </li>
387         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
388         <li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.A">Acknowledgements</a></li>
389         <li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#CORE">Core ABNF of ABNF</a><ul class="toc">
390               <li class="tocline1">B.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.1">Core Rules</a></li>
391               <li class="tocline1">B.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.2">Common Encoding</a></li>
392            </ul>
393         </li>
394         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
395      </ul>
396      <hr class="noprint">
397      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction
398      </h1>
399      <p id="rfc.section.1.p.1">Internet technical specifications often need to define a formal syntax and are free to employ whatever notation their authors
400         deem useful. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among
401         many Internet specifications. It balances compactness and simplicity with reasonable representational power. In the early
402         days of the Arpanet, each specification contained its own definition of ABNF. This included the email specifications, <a href="#RFC733"><cite title="Standard for the format of ARPA network text messages">[RFC733]</cite></a> and then <a href="#RFC822"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a>, which came to be the common citations for defining ABNF. The current document separates those definitions to permit selective
403         reference. Predictably, it also provides some modifications and enhancements.
404      </p>
405      <p id="rfc.section.1.p.2">The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value
406         ranges. <a href="#CORE" title="Core ABNF of ABNF">Appendix&nbsp;B</a> supplies rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications.
407         It is provided as a convenience and is otherwise separate from the meta language defined in the body of this document, and
408         separate from its formal status.
409      </p>
410      <hr class="noprint">
411      <h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;Rule Definition
412      </h1>
413      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;Rule Naming
414      </h2>
415      <p id="rfc.section.2.1.p.1">The name of a rule is simply the name itself, that is, a sequence of characters, beginning with an alphabetic character, and
416         followed by a combination of alphabetics, digits, and hyphens (dashes).
417      </p>
418      <p id="rfc.section.2.1.p.2"> </p>
419      <dl>
420         <dt>NOTE:  </dt>
421         <dd>Rule names are case insensitive.</dd>
422      </dl>
423      <p id="rfc.section.2.1.p.3">The names &lt;rulename&gt;, &lt;Rulename&gt;, &lt;RULENAME&gt;, and &lt;rUlENamE&gt; all refer to the same rule.</p>
424      <p id="rfc.section.2.1.p.4">Unlike original BNF, angle brackets ("&lt;", "&gt;") are not required. However, angle brackets may be used around a rule name whenever
425         their presence facilitates in discerning the use of a rule name. This is typically restricted to rule name references in free-form
426         prose, or to distinguish partial rules that combine into a string not separated by white space, such as shown in the discussion
427         about repetition, below.
428      </p>
429      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;Rule Form
430      </h2>
431      <p id="rfc.section.2.2.p.1">A rule is defined by the following sequence:</p>
432      <div id="rfc.figure.u.1"></div> <pre>
433      name =  elements crlf </pre> <p id="rfc.section.2.2.p.3">where &lt;name&gt; is the name of the rule, &lt;elements&gt; is one or more rule names or terminal specifications, and &lt;crlf&gt; is the end-of-line
434         indicator (carriage return followed by line feed). The equal sign separates the name from the definition of the rule. The
435         elements form a sequence of one or more rule names and/or value definitions, combined according to the various operators defined
436         in this document, such as alternative and repetition.
437      </p>
438      <p id="rfc.section.2.2.p.4">For visual ease, rule definitions are left aligned. When a rule requires multiple lines, the continuation lines are indented.
439         The left alignment and indentation are relative to the first lines of the ABNF rules and need not match the left margin of
440         the document.
441      </p>
442      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;Terminal Values
443      </h2>
444      <p id="rfc.section.2.3.p.1">Rules resolve into a string of terminal values, sometimes called characters. In ABNF, a character is merely a non-negative
445         integer. In certain contexts, a specific mapping (encoding) of values into a character set (such as ASCII) will be specified.
446      </p>
447      <div id="rfc.figure.u.2"></div>
448      <p>Terminals are specified by one or more numeric characters, with the base interpretation of those characters indicated explicitly.
449         The following bases are currently defined:
450      </p>  <pre>
451      b           =  binary
452
453      d           =  decimal
454
455      x           =  hexadecimal </pre> <div id="rfc.figure.u.3"></div>
456      <p>Hence:</p>  <pre>
457      CR          =  %d13
458
459      CR          =  %x0D
460</pre>  <p>respectively specify the decimal and hexadecimal representation of <a href="#US-ASCII"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[US-ASCII]</cite></a> for carriage return.
461      </p>
462      <div id="rfc.figure.u.4"></div>
463      <p>A concatenated string of such values is specified compactly, using a period (".") to indicate a separation of characters within
464         that value. Hence:
465      </p>  <pre>
466      CRLF        =  %d13.10 </pre> <div id="rfc.figure.u.5"></div>
467      <p>ABNF permits the specification of literal text strings directly, enclosed in quotation marks. Hence:</p>  <pre>
468      command     =  "command string" </pre> <p id="rfc.section.2.3.p.6">Literal text strings are interpreted as a concatenated set of printable characters.</p>
469      <p id="rfc.section.2.3.p.7"> </p>
470      <dl>
471         <dt>NOTE:  </dt>
472         <dd>ABNF strings are case insensitive and the character set for these strings is US-ASCII.</dd>
473      </dl>
474      <div id="rfc.figure.u.6"></div>
475      <p>Hence:</p>  <pre>
476      rulename = "abc" </pre> <div id="rfc.figure.u.7"></div>
477      <p>and:</p>  <pre>
478      rulename = "aBc"
479</pre>  <p>will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and "ABC".</p>
480      <p id="rfc.section.2.3.p.10"> </p>
481      <dl class="empty">
482         <dd>To specify a rule that is case sensitive, specify the characters individually.</dd>
483      </dl>
484      <div id="rfc.figure.u.8"></div>
485      <p>For example:</p>  <pre>
486      rulename    =  %d97 %d98 %d99
487</pre> <div id="rfc.figure.u.9"></div>
488      <p>or</p>  <pre>
489      rulename    =  %d97.98.99
490</pre>  <p>will match only the string that comprises only the lowercase characters, abc.</p>
491      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;External Encodings
492      </h2>
493      <p id="rfc.section.2.4.p.1">External representations of terminal value characters will vary according to constraints in the storage or transmission environment.
494         Hence, the same ABNF-based grammar may have multiple external encodings, such as one for a 7-bit US-ASCII environment, another
495         for a binary octet environment, and still a different one when 16-bit Unicode is used. Encoding details are beyond the scope
496         of ABNF, although <a href="#CORE" title="Core ABNF of ABNF">Appendix&nbsp;B</a> provides definitions for a 7-bit US-ASCII environment as has been common to much of the Internet.
497      </p>
498      <p id="rfc.section.2.4.p.2">By separating external encoding from the syntax, it is intended that alternate encoding environments can be used for the same
499         syntax.
500      </p>
501      <hr class="noprint">
502      <h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;Operators
503      </h1>
504      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Concatenation:  Rule1 Rule2
505      </h2>
506      <p id="rfc.section.3.1.p.1">A rule can define a simple, ordered string of values (i.e., a concatenation of contiguous characters) by listing a sequence
507         of rule names. For example:
508      </p>
509      <div id="rfc.figure.u.10"></div> <pre>
510      foo         =  %x61           ; a
511
512      bar         =  %x62           ; b
513
514      mumble      =  foo bar foo </pre> <p id="rfc.section.3.1.p.3">So that the rule &lt;mumble&gt; matches the lowercase string "aba".</p>
515      <p id="rfc.section.3.1.p.4">Linear white space: Concatenation is at the core of the ABNF parsing model. A string of contiguous characters (values) is
516         parsed according to the rules defined in ABNF. For Internet specifications, there is some history of permitting linear white
517         space (space and horizontal tab) to be freely and implicitly interspersed around major constructs, such as delimiting special
518         characters or atomic strings.
519      </p>
520      <p id="rfc.section.3.1.p.5">NOTE: </p>
521      <dl class="empty">
522         <dd>This specification for ABNF does not provide for implicit specification of linear white space.</dd>
523      </dl>
524      <p id="rfc.section.3.1.p.6">Any grammar that wishes to permit linear white space around delimiters or string segments must specify it explicitly. It is
525         often useful to provide for such white space in "core" rules that are then used variously among higher-level rules. The "core"
526         rules might be formed into a lexical analyzer or simply be part of the main ruleset.
527      </p>
528      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="Alternatives" href="#Alternatives">Alternatives:  Rule1 / Rule2</a></h2>
529      <p id="rfc.section.3.2.p.1">Elements separated by a forward slash ("/") are alternatives. Therefore,</p>
530      <div id="rfc.figure.u.11"></div> <pre>
531      foo / bar 
532</pre>  <p>will accept &lt;foo&gt; or &lt;bar&gt;.</p>
533      <p id="rfc.section.3.2.p.3"> </p>
534      <dl>
535         <dt>NOTE:  </dt>
536         <dd>A quoted string containing alphabetic characters is a special form for specifying alternative characters and is interpreted
537            as a non-terminal representing the set of combinatorial strings with the contained characters, in the specified order but
538            with any mixture of upper- and lowercase.
539         </dd>
540      </dl>
541      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="Incremental" href="#Incremental">Incremental Alternatives: Rule1 =/ Rule2</a></h2>
542      <p id="rfc.section.3.3.p.1">It is sometimes convenient to specify a list of alternatives in fragments. That is, an initial rule may match one or more
543         alternatives, with later rule definitions adding to the set of alternatives. This is particularly useful for otherwise independent
544         specifications that derive from the same parent ruleset, such as often occurs with parameter lists. ABNF permits this incremental
545         definition through the construct:
546      </p>
547      <div id="rfc.figure.u.12"></div> <pre>
548      oldrule     =/ additional-alternatives </pre> <div id="rfc.figure.u.13"></div>
549      <p>So that the ruleset</p>  <pre>
550      ruleset     =  alt1 / alt2
551
552      ruleset     =/ alt3
553
554      ruleset     =/ alt4 / alt5 </pre> <div id="rfc.figure.u.14"></div>
555      <p>is the same as specifying</p>  <pre>
556      ruleset     =  alt1 / alt2 / alt3 / alt4 / alt5 </pre> <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="Range" href="#Range">Value Range Alternatives:  %c##-##</a></h2>
557      <div id="rfc.figure.u.15"></div>
558      <p>A range of alternative numeric values can be specified compactly, using a dash ("-") to indicate the range of alternative
559         values. Hence:
560      </p>  <pre>
561      DIGIT       =  %x30-39 </pre> <div id="rfc.figure.u.16"></div>
562      <p>is equivalent to:</p>  <pre>
563      DIGIT       =  "0" / "1" / "2" / "3" / "4" / "5" / "6" /
564
565                     "7" / "8" / "9" </pre> <div id="rfc.figure.u.17"></div>
566      <p>Concatenated numeric values and numeric value ranges cannot be specified in the same string. A numeric value may use the dotted
567         notation for concatenation or it may use the dash notation to specify one value range. Hence, to specify one printable character
568         between end-of-line sequences, the specification could be:
569      </p>  <pre>
570      char-line = %x0D.0A %x20-7E %x0D.0A </pre> <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="Sequence" href="#Sequence">Sequence Group:  (Rule1 Rule2)</a></h2>
571      <div id="rfc.figure.u.18"></div>
572      <p>Elements enclosed in parentheses are treated as a single element, whose contents are strictly ordered. Thus,</p>  <pre>
573      elem (foo / bar) blat
574</pre>  <p>matches (elem foo blat) or (elem bar blat), and</p>
575      <div id="rfc.figure.u.19"></div> <pre>
576      elem foo / bar blat
577</pre>  <p>matches (elem foo) or (bar blat).</p>
578      <p id="rfc.section.3.5.p.3"> </p>
579      <dl>
580         <dt>NOTE:  </dt>
581         <dd>It is strongly advised that grouping notation be used, rather than relying on the proper reading of "bare" alternations, when
582            alternatives consist of multiple rule names or literals.
583         </dd>
584      </dl>
585      <div id="rfc.figure.u.20"></div>
586      <p>Hence, it is recommended that the following form be used:</p>  <pre>
587     (elem foo) / (bar blat)
588</pre>  <p>It will avoid misinterpretation by casual readers.</p>
589      <p id="rfc.section.3.5.p.5">The sequence group notation is also used within free text to set off an element sequence from the prose.</p>
590      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="VarRep" href="#VarRep">Variable Repetition:  *Rule</a></h2>
591      <div id="rfc.figure.u.21"></div>
592      <p>The operator "*" preceding an element indicates repetition. The full form is:</p>  <pre>
593      &lt;a&gt;*&lt;b&gt;element
594</pre>  <p>where &lt;a&gt; and &lt;b&gt; are optional decimal values, indicating at least &lt;a&gt; and at most &lt;b&gt; occurrences of the element.</p>
595      <p id="rfc.section.3.6.p.2">Default values are 0 and infinity so that *&lt;element&gt; allows any number, including zero; 1*&lt;element&gt; requires at least one;
596         3*3&lt;element&gt; allows exactly 3; and 1*2&lt;element&gt; allows one or two.
597      </p>
598      <h2 id="rfc.section.3.7"><a href="#rfc.section.3.7">3.7</a>&nbsp;<a id="SpecRep" href="#SpecRep">Specific Repetition:  nRule</a></h2>
599      <div id="rfc.figure.u.22"></div>
600      <p>A rule of the form:</p>  <pre>
601      &lt;n&gt;element </pre> <div id="rfc.figure.u.23"></div>
602      <p>is equivalent to</p>  <pre>
603      &lt;n&gt;*&lt;n&gt;element </pre> <p id="rfc.section.3.7.p.3">That is, exactly &lt;n&gt; occurrences of &lt;element&gt;. Thus, 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic
604         characters.
605      </p>
606      <h2 id="rfc.section.3.8"><a href="#rfc.section.3.8">3.8</a>&nbsp;<a id="OptSeq" href="#OptSeq">Optional Sequence:  [RULE]</a></h2>
607      <div id="rfc.figure.u.24"></div>
608      <p>Square brackets enclose an optional element sequence:</p>  <pre>
609      [foo bar] </pre> <div id="rfc.figure.u.25"></div>
610      <p>is equivalent to</p>  <pre>
611      *1(foo bar). </pre> <h2 id="rfc.section.3.9"><a href="#rfc.section.3.9">3.9</a>&nbsp;<a id="Comment" href="#Comment">Comment:  ; Comment</a></h2>
612      <p id="rfc.section.3.9.p.1">A semicolon starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel
613         with the specifications.
614      </p>
615      <h2 id="rfc.section.3.10"><a href="#rfc.section.3.10">3.10</a>&nbsp;Operator Precedence
616      </h2>
617      <p id="rfc.section.3.10.p.1">The various mechanisms described above have the following precedence, from highest (binding tightest) at the top, to lowest
618         (loosest) at the bottom:
619      </p>
620      <dl class="empty">
621         <dd>Rule name, prose-val, Terminal value</dd>
622         <dd>Comment</dd>
623         <dd>Value range</dd>
624         <dd>Repetition</dd>
625         <dd>Grouping, Optional</dd>
626         <dd>Concatenation</dd>
627         <dd>Alternative</dd>
628      </dl>
629      <p id="rfc.section.3.10.p.2">Use of the alternative operator, freely mixed with concatenations, can be confusing.</p>
630      <p id="rfc.section.3.10.p.3"> </p>
631      <dl class="empty">
632         <dd>Again, it is recommended that the grouping operator be used to make explicit concatenation groups.</dd>
633      </dl>
634      <hr class="noprint">
635      <h1 id="rfc.section.4" class="np"><a href="#rfc.section.4">4.</a>&nbsp;ABNF Definition of ABNF
636      </h1>
637      <p id="rfc.section.4.p.1"> </p>
638      <dl>
639         <dt>NOTES:</dt>
640         <dd>
641            <ol>
642               <li>This syntax requires a formatting of rules that is relatively strict. Hence, the version of a ruleset included in a specification
643                  might need preprocessing to ensure that it can be interpreted by an ABNF parser.
644               </li>
645               <li>This syntax uses the rules provided in <a href="#CORE" title="Core ABNF of ABNF">Appendix&nbsp;B</a>.
646               </li>
647            </ol>
648         </dd>
649      </dl>
650      <div id="rfc.figure.u.26"></div> <pre class="inline">
651      rulelist       =  1*( rule / (*c-wsp c-nl) )
652
653      rule           =  rulename defined-as elements c-nl
654                             ; continues if next line starts
655                             ;  with white space
656
657      rulename       =  ALPHA *(ALPHA / DIGIT / "-")
658
659      defined-as     =  *c-wsp ("=" / "=/") *c-wsp
660                             ; basic rules definition and
661                             ;  incremental alternatives
662
663      elements       =  alternation *c-wsp
664
665      c-wsp          =  WSP / (c-nl WSP)
666
667      c-nl           =  comment / CRLF
668                             ; comment or newline
669
670      comment        =  ";" *(WSP / VCHAR) CRLF
671
672      alternation    =  concatenation
673                        *(*c-wsp "/" *c-wsp concatenation)
674
675      concatenation  =  repetition *(1*c-wsp repetition)
676
677      repetition     =  [repeat] element
678
679      repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)
680
681      element        =  rulename / group / option /
682                        char-val / num-val / prose-val
683
684      group          =  "(" *c-wsp alternation *c-wsp ")"
685
686      option         =  "[" *c-wsp alternation *c-wsp "]"
687
688      char-val       =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
689                             ; quoted string of SP and VCHAR
690                             ;  without DQUOTE
691
692      num-val        =  "%" (bin-val / dec-val / hex-val)
693
694      bin-val        =  "b" 1*BIT
695                        [ 1*("." 1*BIT) / ("-" 1*BIT) ]
696                             ; series of concatenated bit values
697                             ;  or single ONEOF range
698
699      dec-val        =  "d" 1*DIGIT
700                        [ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]
701
702      hex-val        =  "x" 1*HEXDIG
703                        [ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ]
704
705
706
707
708      prose-val      =  "&lt;" *(%x20-3D / %x3F-7E) "&gt;"
709                             ; bracketed string of SP and VCHAR
710                             ;  without angles
711                             ; prose description, to be used as
712                             ;  last resort </pre> <hr class="noprint">
713      <h1 id="rfc.section.5" class="np"><a href="#rfc.section.5">5.</a>&nbsp;Security Considerations
714      </h1>
715      <p id="rfc.section.5.p.1">Security is truly believed to be irrelevant to this document.</p>
716      <hr class="noprint">
717      <h1 id="rfc.references" class="np"><a id="rfc.section.6" href="#rfc.section.6">6.</a> References
718      </h1>
719      <h2 class="np" id="rfc.references.1"><a href="#rfc.section.6.1" id="rfc.section.6.1">6.1</a> Normative References
720      </h2>
721      <table summary="Normative References"> 
722         <tr>
723            <td class="reference"><b id="US-ASCII">[US-ASCII]</b></td>
724            <td class="top">American National Standards
725               Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI&nbsp;X3.4, 1986.
726            </td>
727         </tr>
728      </table>
729      <h2 id="rfc.references.2"><a href="#rfc.section.6.2" id="rfc.section.6.2">6.2</a> Informative References
730      </h2>
731      <table summary="Informative References">   
732         <tr>
733            <td class="reference"><b id="RFC733">[RFC733]</b></td>
734            <td class="top"><a href="mailto:DCrocker@Rand-Unix" title="The Rand Corporation, Information Sciences&#xA;                     Department">Crocker, D.</a>, <a href="mailto:Vittal@BBN-TenexD" title="Bolt Beranek and Newman Inc. (BBN)">Vittal, J.</a>, <a href="mailto:Pogran@MIT-Multics" title="Massachusets Institute of Technology (MIT),&#xA;                     Laboratory for Computer Science">Pogran, K.</a>, and <a href="mailto:Henderson@BBN-TenexD" title="Bolt Beranek and Newman Inc. (BBN)">D. Henderson</a>, “<a href="http://tools.ietf.org/html/rfc733">Standard for the format of ARPA network text messages</a>”, RFC&nbsp;733, November&nbsp;1977.
735            </td>
736         </tr>
737         <tr>
738            <td class="reference"><b id="RFC822">[RFC822]</b></td>
739            <td class="top"><a href="mailto:DCrocker@UDel-Relay" title="University of Delaware, Dept. of Electrical&#xA;                     Engineering">Crocker, D.H.</a>, “<a href="http://tools.ietf.org/html/rfc822">Standard for the format of ARPA Internet text messages</a>”, STD&nbsp;11, RFC&nbsp;822, August&nbsp;1982.
740            </td>
741         </tr>
742      </table>
743      <hr class="noprint">
744      <h1 id="rfc.authors" class="np"><a href="#rfc.authors">Authors' Addresses</a></h1>
745      <address class="vcard"><span class="vcardline"><span class="fn">Dave Crocker</span>
746            (editor)
747            <span class="n hidden"><span class="family-name">Crocker</span><span class="given-name">Dave</span></span></span><span class="org vcardline">Brandenburg InternetWorking</span><span class="adr"><span class="street-address vcardline">675 Spruce Dr.</span><span class="vcardline"><span class="locality">Sunnyvale</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94086</span></span><span class="country-name vcardline">US</span></span><span class="vcardline tel">Phone: <a href="tel:+1.408.246.8253"><span class="value">+1.408.246.8253</span></a></span><span class="vcardline">Email: <a href="mailto:dcrocker@bbiw.net"><span class="email">dcrocker@bbiw.net</span></a></span></address>
748      <address class="vcard"><span class="vcardline"><span class="fn">Paul Overell</span><span class="n hidden"><span class="family-name">Overell</span><span class="given-name">Paul</span></span></span><span class="org vcardline">THUS plc.</span><span class="adr"><span class="street-address vcardline">1/2 Berkeley Square, </span><span class="street-address vcardline">99 Berkeley Street</span><span class="vcardline"><span class="locality">Glasgow</span>, <span class="postal-code">G3 7HR</span></span><span class="country-name vcardline">UK</span></span><span class="vcardline">Email: <a href="mailto:paul.overell@thus.net"><span class="email">paul.overell@thus.net</span></a></span></address>
749      <hr class="noprint">
750      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;Acknowledgements
751      </h1>
752      <p id="rfc.section.A.p.1">The syntax for ABNF was originally specified in RFC 733. Ken L. Harrenstien, of SRI International, was responsible for re-coding
753         the BNF into an Augmented BNF that makes the representation smaller and easier to understand.
754      </p>
755      <p id="rfc.section.A.p.2">This recent project began as a simple effort to cull out the portion of RFC 822 that has been repeatedly cited by non-email
756         specification writers, namely the description of Augmented BNF. Rather than simply and blindly converting the existing text
757         into a separate document, the working group chose to give careful consideration to the deficiencies, as well as benefits,
758         of the existing specification and related specifications made available over the last 15 years, and therefore to pursue enhancement.
759         This turned the project into something rather more ambitious than was first intended. Interestingly, the result is not massively
760         different from that original, although decisions, such as removing the list notation, came as a surprise.
761      </p>
762      <p id="rfc.section.A.p.3">This "separated" version of the specification was part of the DRUMS working group, with significant contributions from Jerome
763         Abela, Harald Alvestrand, Robert Elz, Roger Fajman, Aviva Garrett, Tom Harsch, Dan Kohn, Bill McQuillan, Keith Moore, Chris
764         Newman, Pete Resnick, and Henning Schulzrinne.
765      </p>
766      <p id="rfc.section.A.p.4">Julian Reschke warrants a special thanks for converting the Draft Standard version to XML source form.</p>
767      <hr class="noprint">
768      <h1 id="rfc.section.B" class="np"><a href="#rfc.section.B">B.</a>&nbsp;<a id="CORE" href="#CORE">Core ABNF of ABNF</a></h1>
769      <p id="rfc.section.B.p.1">This appendix contains some basic rules that are in common use. Basic rules are in uppercase. Note that these rules are only
770         valid for ABNF encoded in 7-bit ASCII or in characters sets that are a superset of 7-bit ASCII.
771      </p>
772      <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Core Rules
773      </h2>
774      <p id="rfc.section.B.1.p.1">Certain basic rules are in uppercase, such as SP, HTAB, CRLF, DIGIT, ALPHA, etc.</p>
775      <div id="rfc.figure.u.27"></div> <pre class="inline">
776      ALPHA          =  %x41-5A / %x61-7A   ; A-Z / a-z
777
778      BIT            =  "0" / "1"
779
780      CHAR           =  %x01-7F
781                             ; any 7-bit US-ASCII character,
782                             ;  excluding NUL
783
784      CR             =  %x0D
785                             ; carriage return
786
787      CRLF           =  CR LF
788                             ; Internet standard newline
789
790      CTL            =  %x00-1F / %x7F
791                             ; controls
792
793      DIGIT          =  %x30-39
794                             ; 0-9
795
796      DQUOTE         =  %x22
797                             ; " (Double Quote)
798
799      HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
800
801      HTAB           =  %x09
802                             ; horizontal tab
803
804      LF             =  %x0A
805                             ; linefeed
806
807      LWSP           =  *(WSP / CRLF WSP)
808                             ; Use of this linear-white-space rule
809                             ;  permits lines containing only white
810                             ;  space that are no longer legal in
811                             ;  mail headers and have caused
812                             ;  interoperability problems in other
813                             ;  contexts.
814                             ; Do not use when defining mail
815                             ;  headers and use with caution in
816                             ;  other contexts.
817
818      OCTET          =  %x00-FF
819                             ; 8 bits of data
820
821      SP             =  %x20
822
823      VCHAR          =  %x21-7E
824                             ; visible (printing) characters
825
826      WSP            =  SP / HTAB
827                             ; white space </pre> <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Common Encoding
828      </h2>
829      <p id="rfc.section.B.2.p.1">Externally, data are represented as "network virtual ASCII" (namely, 7-bit US-ASCII in an 8-bit field), with the high (8th)
830         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
831         side and are sent over the network first.
832      </p>
833      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
834      <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
835         authors retain all their rights.
836      </p>
837      <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
838         HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
839         DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
840         WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
841      </p>
842      <hr class="noprint">
843      <h1 class="np"><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
844      <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
845         be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
846         license under such rights might or might not be available; nor does it represent that it has made any independent effort to
847         identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
848         BCP 79.
849      </p>
850      <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
851         of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
852         of this specification can be obtained from the IETF on-line IPR repository at &lt;<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>&gt;.
853      </p>
854      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
855         rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
856         at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.
857      </p>
858   </body>
859</html>
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