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329</style><link rel="Contents" href="#rfc.toc"><link rel="Author" href="#rfc.authors"><link rel="Copyright" href="#rfc.copyright"><link rel="Chapter" title="1 Introduction" href="#rfc.section.1"><link rel="Chapter" title="2 Rule Definition" href="#rfc.section.2"><link rel="Chapter" title="3 Operators" href="#rfc.section.3"><link rel="Chapter" title="4 ABNF Definition of ABNF" href="#rfc.section.4"><link rel="Chapter" title="5 Security Considerations" href="#rfc.section.5"><link rel="Chapter" href="#rfc.section.6" title="6 References"><link rel="Appendix" title="A Acknowledgements" href="#rfc.section.A"><link rel="Appendix" title="B Core ABNF of ABNF" href="#rfc.section.B"><link rel="Alternate" title="Authorative ASCII version" href="http://www.ietf.org/rfc/rfc5234.txt"><meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.389, 2008-08-20 14:21:35, XSLT vendor: SAXON 6.5.5 from Michael Kay http://saxon.sf.net/"><meta name="keywords" content="ABNF, Augmented, Backus-Naur, Form, electronic, mail"><link rel="schema.DC" href="http://purl.org/dc/elements/1.1/"><meta name="DC.Creator" content="Crocker, D."><meta name="DC.Creator" content="Overell, P."><meta name="DC.Identifier" content="urn:ietf:rfc:5234"><meta name="DC.Date.Issued" scheme="ISO8601" content="2008-01"><meta name="DC.Relation.Replaces" content="urn:ietf:rfc:4234"><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."></head><body><table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1"><tr><td class="header left">Network Working Group</td><td class="header right">D. Crocker, Editor</td></tr><tr><td class="header left">Request for Comments: 5234</td><td class="header right">Brandenburg InternetWorking</td></tr><tr><td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc4234">4234</a></td><td class="header right">P. Overell</td></tr><tr><td class="header left">STD: 68</td><td class="header right">THUS plc.</td></tr><tr><td class="header left">Category: Standards Track</td><td class="header right">January 2008</td></tr></table><p class="title">Augmented BNF for Syntax Specifications: ABNF</p><h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1><p>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 &#8220;Internet Official Protocol Standards&#8221; (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.</p><h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> <p>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.</p> <hr class="noprint"><h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1><ul class="toc"><li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Introduction</a></li><li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Rule Definition</a><ul class="toc"><li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.1">Rule Naming</a></li><li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.2">Rule Form</a></li><li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.3">Terminal Values</a></li><li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.4">External Encodings</a></li></ul></li><li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Operators</a><ul class="toc"><li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Concatenation:  Rule1 Rule2</a></li><li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#Alternatives">Alternatives:  Rule1 / Rule2</a></li><li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#Incremental">Incremental Alternatives: Rule1 =/ Rule2</a></li><li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#Range">Value Range Alternatives:  %c##-##</a></li><li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#Sequence">Sequence Group:  (Rule1 Rule2)</a></li><li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#VarRep">Variable Repetition:  *Rule</a></li><li class="tocline1">3.7&nbsp;&nbsp;&nbsp;<a href="#SpecRep">Specific Repetition:  nRule</a></li><li class="tocline1">3.8&nbsp;&nbsp;&nbsp;<a href="#OptSeq">Optional Sequence:  [RULE]</a></li><li class="tocline1">3.9&nbsp;&nbsp;&nbsp;<a href="#Comment">Comment:  ; Comment</a></li><li class="tocline1">3.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.10">Operator Precedence</a></li></ul></li><li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4">ABNF Definition of ABNF</a></li><li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Security Considerations</a></li><li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc"><li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li><li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li></ul></li><li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li><li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.A">Acknowledgements</a></li><li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#CORE">Core ABNF of ABNF</a><ul class="toc"><li class="tocline1">B.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.1">Core Rules</a></li><li class="tocline1">B.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.2">Common Encoding</a></li></ul></li><li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li></ul><hr class="noprint"><h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction</h1><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 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, <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 reference. Predictably, it also provides some modifications and enhancements.</p><p id="rfc.section.1.p.2">The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value 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. 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.</p><hr class="noprint"><h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;Rule Definition</h1><h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;Rule Naming</h2><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 followed by a combination of alphabetics, digits, and hyphens (dashes).</p><p id="rfc.section.2.1.p.2"> </p><dl><dt>NOTE:  </dt><dd>Rule names are case insensitive.</dd></dl><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><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 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.</p><h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;Rule Form</h2><p id="rfc.section.2.2.p.1">A rule is defined by the following sequence:</p><div id="rfc.figure.u.1"></div> <pre>
330      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 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.</p><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. 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.</p><h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;Terminal Values</h2><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 integer. In certain contexts, a specific mapping (encoding) of values into a character set (such as ASCII) will be specified.</p><div id="rfc.figure.u.2"></div> <p>Terminals are specified by one or more numeric characters, with the base interpretation of those characters indicated explicitly. The following bases are currently defined:</p>  <pre>
331      b           =  binary
332
333      d           =  decimal
334
335      x           =  hexadecimal </pre> <div id="rfc.figure.u.3"></div> <p>Hence:</p>  <pre>
336      CR          =  %d13
337
338      CR          =  %x0D
339</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.</p> <div id="rfc.figure.u.4"></div> <p>A concatenated string of such values is specified compactly, using a period (".") to indicate a separation of characters within that value. Hence:</p>  <pre>
340      CRLF        =  %d13.10 </pre> <div id="rfc.figure.u.5"></div> <p>ABNF permits the specification of literal text strings directly, enclosed in quotation marks. Hence:</p>  <pre>
341      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><p id="rfc.section.2.3.p.7"> </p><dl><dt>NOTE:  </dt><dd>ABNF strings are case insensitive and the character set for these strings is US-ASCII.</dd></dl><div id="rfc.figure.u.6"></div> <p>Hence:</p>  <pre>
342      rulename = "abc" </pre> <div id="rfc.figure.u.7"></div> <p>and:</p>  <pre>
343      rulename = "aBc"
344</pre>  <p>will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and "ABC".</p> <p id="rfc.section.2.3.p.10"> </p><dl class="empty"><dd>To specify a rule that is case sensitive, specify the characters individually.</dd></dl><div id="rfc.figure.u.8"></div> <p>For example:</p>  <pre>
345      rulename    =  %d97 %d98 %d99
346</pre> <div id="rfc.figure.u.9"></div> <p>or</p>  <pre>
347      rulename    =  %d97.98.99
348</pre>  <p>will match only the string that comprises only the lowercase characters, abc.</p> <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;External Encodings</h2><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. 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 <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.</p><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 syntax.</p><hr class="noprint"><h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;Operators</h1><h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Concatenation:  Rule1 Rule2</h2><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 of rule names. For example:</p><div id="rfc.figure.u.10"></div> <pre>
349      foo         =  %x61           ; a
350
351      bar         =  %x62           ; b
352
353      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><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 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.</p><p id="rfc.section.3.1.p.5">NOTE: </p><dl class="empty"><dd>This specification for ABNF does not provide for implicit specification of linear white space.</dd></dl><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 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.</p><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><p id="rfc.section.3.2.p.1">Elements separated by a forward slash ("/") are alternatives. Therefore,</p><div id="rfc.figure.u.11"></div> <pre>
354      foo / bar 
355</pre>  <p>will accept &lt;foo&gt; or &lt;bar&gt;.</p> <p id="rfc.section.3.2.p.3"> </p><dl><dt>NOTE:  </dt><dd>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 lowercase.</dd></dl><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><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 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 ruleset, such as often occurs with parameter lists. ABNF permits this incremental definition through the construct:</p><div id="rfc.figure.u.12"></div> <pre>
356      oldrule     =/ additional-alternatives </pre> <div id="rfc.figure.u.13"></div> <p>So that the ruleset</p>  <pre>
357      ruleset     =  alt1 / alt2
358
359      ruleset     =/ alt3
360
361      ruleset     =/ alt4 / alt5 </pre> <div id="rfc.figure.u.14"></div> <p>is the same as specifying</p>  <pre>
362      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><div id="rfc.figure.u.15"></div> <p>A range of alternative numeric values can be specified compactly, using a dash ("-") to indicate the range of alternative values. Hence:</p>  <pre>
363      DIGIT       =  %x30-39 </pre> <div id="rfc.figure.u.16"></div> <p>is equivalent to:</p>  <pre>
364      DIGIT       =  "0" / "1" / "2" / "3" / "4" / "5" / "6" /
365
366                     "7" / "8" / "9" </pre> <div id="rfc.figure.u.17"></div> <p>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 specify one value range. Hence, to specify one printable character between end-of-line sequences, the specification could be:</p>  <pre>
367      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><div id="rfc.figure.u.18"></div> <p>Elements enclosed in parentheses are treated as a single element, whose contents are strictly ordered. Thus,</p>  <pre>
368      elem (foo / bar) blat
369</pre>  <p>matches (elem foo blat) or (elem bar blat), and</p> <div id="rfc.figure.u.19"></div> <pre>
370      elem foo / bar blat
371</pre>  <p>matches (elem foo) or (bar blat).</p> <p id="rfc.section.3.5.p.3"> </p><dl><dt>NOTE:  </dt><dd>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.</dd></dl><div id="rfc.figure.u.20"></div> <p>Hence, it is recommended that the following form be used:</p>  <pre>
372     (elem foo) / (bar blat)
373</pre>  <p>It will avoid misinterpretation by casual readers.</p> <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><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><div id="rfc.figure.u.21"></div> <p>The operator "*" preceding an element indicates repetition. The full form is:</p>  <pre>
374      &lt;a&gt;*&lt;b&gt;element
375</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> <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; 3*3&lt;element&gt; allows exactly 3; and 1*2&lt;element&gt; allows one or two.</p><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><div id="rfc.figure.u.22"></div> <p>A rule of the form:</p>  <pre>
376      &lt;n&gt;element </pre> <div id="rfc.figure.u.23"></div> <p>is equivalent to</p>  <pre>
377      &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 characters.</p><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><div id="rfc.figure.u.24"></div> <p>Square brackets enclose an optional element sequence:</p>  <pre>
378      [foo bar] </pre> <div id="rfc.figure.u.25"></div> <p>is equivalent to</p>  <pre>
379      *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><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 with the specifications.</p><h2 id="rfc.section.3.10"><a href="#rfc.section.3.10">3.10</a>&nbsp;Operator Precedence</h2><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 (loosest) at the bottom: </p><dl class="empty"><dd>Rule name, prose-val, Terminal value</dd><dd>Comment</dd><dd>Value range</dd><dd>Repetition</dd><dd>Grouping, Optional</dd><dd>Concatenation</dd><dd>Alternative</dd></dl><p id="rfc.section.3.10.p.2">Use of the alternative operator, freely mixed with concatenations, can be confusing.</p><p id="rfc.section.3.10.p.3"> </p><dl class="empty"><dd>Again, it is recommended that the grouping operator be used to make explicit concatenation groups.</dd></dl><hr class="noprint"><h1 id="rfc.section.4" class="np"><a href="#rfc.section.4">4.</a>&nbsp;ABNF Definition of ABNF</h1><p id="rfc.section.4.p.1"> </p><dl><dt>NOTES:</dt><dd> <ol><li>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.</li><li>This syntax uses the rules provided in <a href="#CORE" title="Core ABNF of ABNF">Appendix&nbsp;B</a>.</li></ol> </dd></dl><div id="rfc.figure.u.26"></div> <pre class="inline">
380      rulelist       =  1*( rule / (*c-wsp c-nl) )
381
382      rule           =  rulename defined-as elements c-nl
383                             ; continues if next line starts
384                             ;  with white space
385
386      rulename       =  ALPHA *(ALPHA / DIGIT / "-")
387
388      defined-as     =  *c-wsp ("=" / "=/") *c-wsp
389                             ; basic rules definition and
390                             ;  incremental alternatives
391
392      elements       =  alternation *c-wsp
393
394      c-wsp          =  WSP / (c-nl WSP)
395
396      c-nl           =  comment / CRLF
397                             ; comment or newline
398
399      comment        =  ";" *(WSP / VCHAR) CRLF
400
401      alternation    =  concatenation
402                        *(*c-wsp "/" *c-wsp concatenation)
403
404      concatenation  =  repetition *(1*c-wsp repetition)
405
406      repetition     =  [repeat] element
407
408      repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)
409
410      element        =  rulename / group / option /
411                        char-val / num-val / prose-val
412
413      group          =  "(" *c-wsp alternation *c-wsp ")"
414
415      option         =  "[" *c-wsp alternation *c-wsp "]"
416
417      char-val       =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
418                             ; quoted string of SP and VCHAR
419                             ;  without DQUOTE
420
421      num-val        =  "%" (bin-val / dec-val / hex-val)
422
423      bin-val        =  "b" 1*BIT
424                        [ 1*("." 1*BIT) / ("-" 1*BIT) ]
425                             ; series of concatenated bit values
426                             ;  or single ONEOF range
427
428      dec-val        =  "d" 1*DIGIT
429                        [ 1*("." 1*DIGIT) / ("-" 1*DIGIT) ]
430
431      hex-val        =  "x" 1*HEXDIG
432                        [ 1*("." 1*HEXDIG) / ("-" 1*HEXDIG) ]
433
434
435
436
437      prose-val      =  "&lt;" *(%x20-3D / %x3F-7E) "&gt;"
438                             ; bracketed string of SP and VCHAR
439                             ;  without angles
440                             ; prose description, to be used as
441                             ;  last resort </pre> <hr class="noprint"><h1 id="rfc.section.5" class="np"><a href="#rfc.section.5">5.</a>&nbsp;Security Considerations</h1><p id="rfc.section.5.p.1">Security is truly believed to be irrelevant to this document.</p><hr class="noprint"><h1 id="rfc.references" class="np"><a id="rfc.section.6" href="#rfc.section.6">6.</a> References</h1><h2 class="np" id="rfc.references.1"><a href="#rfc.section.6.1" id="rfc.section.6.1">6.1</a> Normative References</h2><table summary="Normative References">  <tr><td class="reference"><b id="US-ASCII">[US-ASCII]</b></td><td class="top">American National Standards
442                  Institute, &#8220;Coded Character Set -- 7-bit American Standard Code for
443                  Information Interchange&#8221;, ANSI&nbsp;X3.4, 1986.</td></tr></table><h2 id="rfc.references.2"><a href="#rfc.section.6.2" id="rfc.section.6.2">6.2</a> Informative References</h2><table summary="Informative References">    <tr><td class="reference"><b id="RFC733">[RFC733]</b></td><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>, &#8220;<a href="http://tools.ietf.org/html/rfc733">Standard for the format of ARPA network text messages</a>&#8221;, RFC&nbsp;733, November&nbsp;1977.</td></tr><tr><td class="reference"><b id="RFC822">[RFC822]</b></td><td class="top"><a href="mailto:DCrocker@UDel-Relay" title="University of Delaware, Dept. of Electrical&#xA;                     Engineering">Crocker, D.H.</a>, &#8220;<a href="http://tools.ietf.org/html/rfc822">Standard for the format of ARPA Internet text messages</a>&#8221;, STD&nbsp;11, RFC&nbsp;822, August&nbsp;1982.</td></tr></table><hr class="noprint"><h1 id="rfc.authors" class="np"><a href="#rfc.authors">Authors' Addresses</a></h1><address class="vcard"><span class="vcardline"><span class="fn">Dave Crocker</span>
444          (editor)
445        <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><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><hr class="noprint"><h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;Acknowledgements</h1><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 the BNF into an Augmented BNF that makes the representation smaller and easier to understand.</p><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 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.</p><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 Abela, Harald Alvestrand, Robert Elz, Roger Fajman, Aviva Garrett, Tom Harsch, Dan Kohn, Bill McQuillan, Keith Moore, Chris Newman, Pete Resnick, and Henning Schulzrinne.</p><p id="rfc.section.A.p.4">Julian Reschke warrants a special thanks for converting the Draft Standard version to XML source form.</p><hr class="noprint"><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><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 valid for ABNF encoded in 7-bit ASCII or in characters sets that are a superset of 7-bit ASCII.</p><h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Core Rules</h2><p id="rfc.section.B.1.p.1">Certain basic rules are in uppercase, such as SP, HTAB, CRLF, DIGIT, ALPHA, etc.</p><div id="rfc.figure.u.27"></div> <pre class="inline">
446      ALPHA          =  %x41-5A / %x61-7A   ; A-Z / a-z
447
448      BIT            =  "0" / "1"
449
450      CHAR           =  %x01-7F
451                             ; any 7-bit US-ASCII character,
452                             ;  excluding NUL
453
454      CR             =  %x0D
455                             ; carriage return
456
457      CRLF           =  CR LF
458                             ; Internet standard newline
459
460      CTL            =  %x00-1F / %x7F
461                             ; controls
462
463      DIGIT          =  %x30-39
464                             ; 0-9
465
466      DQUOTE         =  %x22
467                             ; " (Double Quote)
468
469      HEXDIG         =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
470
471      HTAB           =  %x09
472                             ; horizontal tab
473
474      LF             =  %x0A
475                             ; linefeed
476
477      LWSP           =  *(WSP / CRLF WSP)
478                             ; Use of this linear-white-space rule
479                             ;  permits lines containing only white
480                             ;  space that are no longer legal in
481                             ;  mail headers and have caused
482                             ;  interoperability problems in other
483                             ;  contexts.
484                             ; Do not use when defining mail
485                             ;  headers and use with caution in
486                             ;  other contexts.
487
488      OCTET          =  %x00-FF
489                             ; 8 bits of data
490
491      SP             =  %x20
492
493      VCHAR          =  %x21-7E
494                             ; visible (printing) characters
495
496      WSP            =  SP / HTAB
497                             ; white space </pre> <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Common Encoding</h2><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) 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.</p><h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1><p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.</p><p>This document and the information contained herein are provided on an &#8220;AS IS&#8221; basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST 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.</p><hr class="noprint"><h1 class="np"><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1><p>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 in BCP 78 and BCP 79.</p><p>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 at &lt;<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>&gt;.</p><p>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 <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.</p></body></html>
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