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<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="EN"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Extensible Markup Language (XML) 1.0 (Fifth Edition)</title><style type="text/css"> code { font-family: monospace; } div.constraint, div.issue, div.note, div.notice { margin-left: 2em; } ol.enumar { list-style-type: decimal; } ol.enumla { list-style-type: lower-alpha; } ol.enumlr { list-style-type: lower-roman; } ol.enumua { list-style-type: upper-alpha; } ol.enumur { list-style-type: upper-roman; } div.exampleInner pre { margin-left: 1em; margin-top: 0em; margin-bottom: 0em} div.exampleOuter {border: 4px double gray; margin: 0em; padding: 0em} div.exampleInner { background-color: #d5dee3; border-top-width: 4px; border-top-style: double; border-top-color: #d3d3d3; border-bottom-width: 4px; border-bottom-style: double; border-bottom-color: #d3d3d3; padding: 4px; margin: 0em } div.exampleWrapper { margin: 4px } div.exampleHeader { font-weight: bold; margin: 4px} em.rfc2119 { text-transform: lowercase; font-variant: small-caps; font-style: normal; } </style><link rel="stylesheet" type="text/css" href="https://www.w3.org/StyleSheets/TR/W3C-REC.css" /></head><body><div class="head"><p><a href="https://www.w3.org/"><img src="https://www.w3.org/Icons/w3c_home" alt="W3C" height="48" width="72" /></a></p> <h1><a name="title" id="title"/>Extensible Markup Language (XML) 1.0 (Fifth Edition)</h1> <h2><a name="w3c-doctype" id="w3c-doctype"/>W3C Recommendation 26 November 2008</h2> <div id="notice-20130207" style="border: solid black 1px; padding: 0.5em; background: #FFB;"> <p><strong>Note:</strong> On 7 February 2013, this specification was modified in place to replace broken links to RFC4646 and RFC4647.</p> </div> <dl><dt>This version:</dt><dd> <a href="https://www.w3.org/TR/2008/REC-xml-20081126/">http://www.w3.org/TR/2008/REC-xml-20081126/</a> </dd><dt>Latest version:</dt><dd> <a href="https://www.w3.org/TR/xml/">http://www.w3.org/TR/xml/</a> </dd><dt>Previous versions:</dt><dd> <a href="https://www.w3.org/TR/2008/PER-xml-20080205/">http://www.w3.org/TR/2008/PER-xml-20080205/</a> <br /> <a href="https://www.w3.org/TR/2006/REC-xml-20060816/">http://www.w3.org/TR/2006/REC-xml-20060816/</a> </dd><dt>Editors:</dt><dd>Tim Bray, Textuality and Netscape <a href="mailto:tbray@textuality.com"><tbray@textuality.com></a></dd><dd>Jean Paoli, Microsoft <a href="mailto:jeanpa@microsoft.com"><jeanpa@microsoft.com></a></dd><dd>C. M. Sperberg-McQueen, W3C <a href="mailto:cmsmcq@w3.org"><cmsmcq@w3.org></a></dd><dd>Eve Maler, Sun Microsystems, Inc. <a href="mailto:elm@east.sun.com"><eve.maler@east.sun.com></a></dd><dd>François Yergeau</dd></dl><p>Please refer to the <a href="https://www.w3.org/XML/xml-V10-5e-errata"><strong>errata</strong></a> for this document, which may include some normative corrections.</p><p>The <a href="https://www.w3.org/XML/xml-V10-4e-errata">previous errata</a> for this document, are also available.</p><p>See also <a href="https://www.w3.org/2003/03/Translations/byTechnology?technology=xml"><strong>translations</strong></a>.</p><p>This document is also available in these non-normative formats: <a href="REC-xml-20081126.xml">XML</a> and <a href="REC-xml-20081126-review.html">XHTML with color-coded revision indicators</a>.</p><p class="copyright"><a href="https://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2008 <a href="https://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>, <a href="http://www.ercim.org/"><acronym title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a href="https://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.</p></div><hr/><div> <h2><a name="abstract" id="abstract"/>Abstract</h2><p>The Extensible Markup Language (XML) is a subset of SGML that is completely described in this document. Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.</p></div><div> <h2><a name="status" id="status"/>Status of this Document</h2><p><em>This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the <a href="https://www.w3.org/TR/">W3C technical reports index</a> at http://www.w3.org/TR/.</em></p><p>This document specifies a syntax created by subsetting an existing, widely used international text processing standard (Standard Generalized Markup Language, ISO 8879:1986(E) as amended and corrected) for use on the World Wide Web. It is a product of the <a href="https://www.w3.org/XML/Core/">XML Core Working Group</a> as part of the <a href="https://www.w3.org/XML/Activity">XML Activity</a>. The English version of this specification is the only normative version. However, for translations of this document, see <a href="https://www.w3.org/2003/03/Translations/byTechnology?technology=xml">http://www.w3.org/2003/03/Translations/byTechnology?technology=xml</a>.</p><p>This document is a <a href="https://www.w3.org/2005/10/Process-20051014/tr.html#q74">W3C Recommendation</a>. This fifth edition is <em>not</em> a new version of XML. As a convenience to readers, it incorporates the changes dictated by the accumulated errata (available at <a href="https://www.w3.org/XML/xml-V10-4e-errata">http://www.w3.org/XML/xml-V10-4e-errata</a>) to the <a href="https://www.w3.org/TR/2006/REC-xml-20060816/">Fourth Edition of XML 1.0, dated 16 August 2006</a>. In particular, erratum <a href="https://www.w3.org/XML/xml-V10-4e-errata#E09">[E09]</a> relaxes the restrictions on element and attribute names, thereby providing in XML 1.0 the major end user benefit currently achievable only by using XML 1.1. As a consequence, many possible documents which were not well-formed according to previous editions of this specification are now well-formed, and previously invalid documents using the newly-allowed name characters in, for example, ID attributes, are now valid.</p><p>This edition supersedes the previous <a href="https://www.w3.org/TR/2006/REC-xml-20060816/">W3C Recommendation of 16 August 2006</a>.</p><p>Please report errors in this document to the public <a href="mailto:xml-editor@w3.org">xml-editor@w3.org</a> mail list; public <a href="http://lists.w3.org/Archives/Public/xml-editor/">archives</a> are available. For the convenience of readers, an <a href="REC-xml-20081126-review.html">XHTML version with color-coded revision indicators</a> is also provided; this version highlights each change due to an erratum published in the <a href="https://www.w3.org/XML/xml-V10-4e-errata">errata list</a> for the previous edition, together with a link to the particular erratum in that list. Most of the errata in the list provide a rationale for the change. The errata list for this fifth edition is available at <a href="https://www.w3.org/XML/xml-V10-5e-errata">http://www.w3.org/XML/xml-V10-5e-errata</a>.</p><p>An implementation report is available at <a href="https://www.w3.org/XML/2008/01/xml10-5e-implementation.html">http://www.w3.org/XML/2008/01/xml10-5e-implementation.html</a>. A <a href="https://www.w3.org/XML/Test/">Test Suite</a> is maintained to help assessing conformance to this specification.</p><p>This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.</p><p>W3C maintains a <a href="https://www.w3.org/2002/08/xmlcore-IPR-statements" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains <a href="https://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.</p></div><div class="toc"> <h2><a name="contents" id="contents"/>Table of Contents</h2><p class="toc">1 <a href="#sec-intro">Introduction</a><br/> 1.1 <a href="#sec-origin-goals">Origin and Goals</a><br/> 1.2 <a href="#sec-terminology">Terminology</a><br/> 2 <a href="#sec-documents">Documents</a><br/> 2.1 <a href="#sec-well-formed">Well-Formed XML Documents</a><br/> 2.2 <a href="#charsets">Characters</a><br/> 2.3 <a href="#sec-common-syn">Common Syntactic Constructs</a><br/> 2.4 <a href="#syntax">Character Data and Markup</a><br/> 2.5 <a href="#sec-comments">Comments</a><br/> 2.6 <a href="#sec-pi">Processing Instructions</a><br/> 2.7 <a href="#sec-cdata-sect">CDATA Sections</a><br/> 2.8 <a href="#sec-prolog-dtd">Prolog and Document Type Declaration</a><br/> 2.9 <a href="#sec-rmd">Standalone Document Declaration</a><br/> 2.10 <a href="#sec-white-space">White Space Handling</a><br/> 2.11 <a href="#sec-line-ends">End-of-Line Handling</a><br/> 2.12 <a href="#sec-lang-tag">Language Identification</a><br/> 3 <a href="#sec-logical-struct">Logical Structures</a><br/> 3.1 <a href="#sec-starttags">Start-Tags, End-Tags, and Empty-Element Tags</a><br/> 3.2 <a href="#elemdecls">Element Type Declarations</a><br/> 3.2.1 <a href="#sec-element-content">Element Content</a><br/> 3.2.2 <a href="#sec-mixed-content">Mixed Content</a><br/> 3.3 <a href="#attdecls">Attribute-List Declarations</a><br/> 3.3.1 <a href="#sec-attribute-types">Attribute Types</a><br/> 3.3.2 <a href="#sec-attr-defaults">Attribute Defaults</a><br/> 3.3.3 <a href="#AVNormalize">Attribute-Value Normalization</a><br/> 3.4 <a href="#sec-condition-sect">Conditional Sections</a><br/> 4 <a href="#sec-physical-struct">Physical Structures</a><br/> 4.1 <a href="#sec-references">Character and Entity References</a><br/> 4.2 <a href="#sec-entity-decl">Entity Declarations</a><br/> 4.2.1 <a href="#sec-internal-ent">Internal Entities</a><br/> 4.2.2 <a href="#sec-external-ent">External Entities</a><br/> 4.3 <a href="#TextEntities">Parsed Entities</a><br/> 4.3.1 <a href="#sec-TextDecl">The Text Declaration</a><br/> 4.3.2 <a href="#wf-entities">Well-Formed Parsed Entities</a><br/> 4.3.3 <a href="#charencoding">Character Encoding in Entities</a><br/> 4.4 <a href="#entproc">XML Processor Treatment of Entities and References</a><br/> 4.4.1 <a href="#not-recognized">Not Recognized</a><br/> 4.4.2 <a href="#included">Included</a><br/> 4.4.3 <a href="#include-if-valid">Included If Validating</a><br/> 4.4.4 <a href="#forbidden">Forbidden</a><br/> 4.4.5 <a href="#inliteral">Included in Literal</a><br/> 4.4.6 <a href="#notify">Notify</a><br/> 4.4.7 <a href="#bypass">Bypassed</a><br/> 4.4.8 <a href="#as-PE">Included as PE</a><br/> 4.4.9 <a href="#error">Error</a><br/> 4.5 <a href="#intern-replacement">Construction of Entity Replacement Text</a><br/> 4.6 <a href="#sec-predefined-ent">Predefined Entities</a><br/> 4.7 <a href="#Notations">Notation Declarations</a><br/> 4.8 <a href="#sec-doc-entity">Document Entity</a><br/> 5 <a href="#sec-conformance">Conformance</a><br/> 5.1 <a href="#proc-types">Validating and Non-Validating Processors</a><br/> 5.2 <a href="#safe-behavior">Using XML Processors</a><br/> 6 <a href="#sec-notation">Notation</a><br/> </p> <h3><a name="appendices" id="appendices"/>Appendices</h3><p class="toc">A <a href="#sec-bibliography">References</a><br/> A.1 <a href="#sec-existing-stds">Normative References</a><br/> A.2 <a href="#null">Other References</a><br/> B <a href="#CharClasses">Character Classes</a><br/> C <a href="#sec-xml-and-sgml">XML and SGML</a> (Non-Normative)<br/> D <a href="#sec-entexpand">Expansion of Entity and Character References</a> (Non-Normative)<br/> E <a href="#determinism">Deterministic Content Models</a> (Non-Normative)<br/> F <a href="#sec-guessing">Autodetection of Character Encodings</a> (Non-Normative)<br/> F.1 <a href="#sec-guessing-no-ext-info">Detection Without External Encoding Information</a><br/> F.2 <a href="#sec-guessing-with-ext-info">Priorities in the Presence of External Encoding Information</a><br/> G <a href="#sec-xml-wg">W3C XML Working Group</a> (Non-Normative)<br/> H <a href="#sec-core-wg">W3C XML Core Working Group</a> (Non-Normative)<br/> I <a href="#prod-notes">Production Notes</a> (Non-Normative)<br/> J <a href="#sec-suggested-names">Suggestions for XML Names</a> (Non-Normative)<br/> </p></div><hr/><div class="body"><div class="div1"> <h2><a name="sec-intro" id="sec-intro"/>1 Introduction</h2><p>Extensible Markup Language, abbreviated XML, describes a class of data objects called <a title="XML Document" href="#dt-xml-doc">XML documents</a> and partially describes the behavior of computer programs which process them. XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language <a href="#ISO8879">[ISO 8879]</a>. By construction, XML documents are conforming SGML documents.</p><p>XML documents are made up of storage units called <a title="Entity" href="#dt-entity">entities</a>, which contain either parsed or unparsed data. Parsed data is made up of <a title="Character" href="#dt-character">characters</a>, some of which form <a title="Character Data" href="#dt-chardata">character data</a>, and some of which form <a title="Markup" href="#dt-markup">markup</a>. Markup encodes a description of the document's storage layout and logical structure. XML provides a mechanism to impose constraints on the storage layout and logical structure.</p><p> [<a name="dt-xml-proc" id="dt-xml-proc" title="XML Processor">Definition</a>: A software module called an <b>XML processor</b> is used to read XML documents and provide access to their content and structure.] [<a name="dt-app" id="dt-app" title="Application">Definition</a>: It is assumed that an XML processor is doing its work on behalf of another module, called the <b>application</b>.] This specification describes the required behavior of an XML processor in terms of how it must read XML data and the information it must provide to the application.</p><div class="div2"> <h3><a name="sec-origin-goals" id="sec-origin-goals"/>1.1 Origin and Goals</h3><p>XML was developed by an XML Working Group (originally known as the SGML Editorial Review Board) formed under the auspices of the World Wide Web Consortium (W3C) in 1996. It was chaired by Jon Bosak of Sun Microsystems with the active participation of an XML Special Interest Group (previously known as the SGML Working Group) also organized by the W3C. The membership of the XML Working Group is given in an appendix. Dan Connolly served as the Working Group's contact with the W3C.</p><p>The design goals for XML are:</p><ol class="enumar"><li><p>XML shall be straightforwardly usable over the Internet.</p></li><li><p>XML shall support a wide variety of applications.</p></li><li><p>XML shall be compatible with SGML.</p></li><li><p>It shall be easy to write programs which process XML documents.</p></li><li><p>The number of optional features in XML is to be kept to the absolute minimum, ideally zero.</p></li><li><p>XML documents should be human-legible and reasonably clear.</p></li><li><p>The XML design should be prepared quickly.</p></li><li><p>The design of XML shall be formal and concise.</p></li><li><p>XML documents shall be easy to create.</p></li><li><p>Terseness in XML markup is of minimal importance.</p></li></ol><p>This specification, together with associated standards (Unicode <a href="#Unicode">[Unicode]</a> and ISO/IEC 10646 <a href="#ISO10646">[ISO/IEC 10646]</a> for characters, Internet <span>BCP 47</span> <a href="#RFC1766">[IETF BCP 47]</a> <span>and the Language Subtag Registry <a href="#IANA-LANGCODES">[IANA-LANGCODES]</a></span> for language identification tags), provides all the information necessary to understand XML Version 1.0 and construct computer programs to process it.</p><p>This version of the XML specification may be distributed freely, as long as all text and legal notices remain intact.</p></div><div class="div2"> <h3><a name="sec-terminology" id="sec-terminology"/>1.2 Terminology</h3><p>The terminology used to describe XML documents is defined in the body of this specification. The key words <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">SHALL</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">SHALL NOT</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD NOT</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">RECOMMENDED</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em>, and <em class="rfc2119" title="Keyword in RFC 2119 context">OPTIONAL</em>, when <em class="rfc2119" title="Keyword in RFC 2119 context">EMPHASIZED</em>, are to be interpreted as described in <a href="#rfc2119">[IETF RFC 2119]</a>. In addition, the terms defined in the following list are used in building those definitions and in describing the actions of an XML processor:</p><dl><dt class="label">error</dt><dd><p> [<a name="dt-error" id="dt-error" title="Error">Definition</a>: A violation of the rules of this specification; results are undefined. Unless otherwise specified, failure to observe a prescription of this specification indicated by one of the keywords <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em>, <em class="rfc2119" title="Keyword in RFC 2119 context">SHALL</em> and <em class="rfc2119" title="Keyword in RFC 2119 context">SHALL NOT</em> is an error. Conforming software <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> detect and report an error and <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> recover from it.] </p></dd><dt class="label">fatal error</dt><dd><p> [<a name="dt-fatal" id="dt-fatal" title="Fatal Error">Definition</a>: An error which a conforming <a title="XML Processor" href="#dt-xml-proc">XML processor</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> detect and report to the application. After encountering a fatal error, the processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> continue processing the data to search for further errors and <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> report such errors to the application. In order to support correction of errors, the processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> make unprocessed data from the document (with intermingled character data and markup) available to the application. Once a fatal error is detected, however, the processor <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> continue normal processing (i.e., it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> continue to pass character data and information about the document's logical structure to the application in the normal way).] </p></dd><dt class="label">at user option</dt><dd><p> [<a name="dt-atuseroption" id="dt-atuseroption" title="At user option">Definition</a>: Conforming software <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> or <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> (depending on the modal verb in the sentence) behave as described; if it does, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> provide users a means to enable or disable the behavior described.] </p></dd><dt class="label">validity constraint</dt><dd><p> [<a name="dt-vc" id="dt-vc" title="Validity constraint">Definition</a>: A rule which applies to all <a title="Validity" href="#dt-valid">valid</a> XML documents. Violations of validity constraints are errors; they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em>, at user option, be reported by <a title="Validating Processor" href="#dt-validating">validating XML processors</a>.] </p></dd><dt class="label">well-formedness constraint</dt><dd><p> [<a name="dt-wfc" id="dt-wfc" title="Well-formedness constraint">Definition</a>: A rule which applies to all <a title="Well-Formed" href="#dt-wellformed">well-formed</a> XML documents. Violations of well-formedness constraints are <a title="Fatal Error" href="#dt-fatal">fatal errors</a>.] </p></dd><dt class="label">match</dt><dd><p> [<a name="dt-match" id="dt-match" title="match">Definition</a>: (Of strings or names:) Two strings or names being compared are identical. Characters with multiple possible representations in ISO/IEC 10646 (e.g. characters with both precomposed and base+diacritic forms) match only if they have the same representation in both strings. No case folding is performed. (Of strings and rules in the grammar:) A string matches a grammatical production if it belongs to the language generated by that production. (Of content and content models:) An element matches its declaration when it conforms in the fashion described in the constraint <b>[VC: <a href="#elementvalid">Element Valid</a>]</b>.] </p></dd><dt class="label">for compatibility</dt><dd><p> [<a name="dt-compat" id="dt-compat" title="For Compatibility">Definition</a>: Marks a sentence describing a feature of XML included solely to ensure that XML remains compatible with SGML.] </p></dd><dt class="label">for interoperability</dt><dd><p> [<a name="dt-interop" id="dt-interop" title="For interoperability">Definition</a>: Marks a sentence describing a non-binding recommendation included to increase the chances that XML documents can be processed by the existing installed base of SGML processors which predate the WebSGML Adaptations Annex to ISO 8879.] </p></dd></dl><p> </p></div></div><div class="div1"> <h2><a name="sec-documents" id="sec-documents"/>2 Documents</h2><p> [<a name="dt-xml-doc" id="dt-xml-doc" title="XML Document">Definition</a>: A data object is an <b>XML document</b> if it is <a title="Well-Formed" href="#dt-wellformed">well-formed</a>, as defined in this specification. In addition, the XML document is <a title="Validity" href="#dt-valid">valid</a> if it meets certain further constraints.] </p><p>Each XML document has both a logical and a physical structure. Physically, the document is composed of units called <a title="Entity" href="#dt-entity">entities</a>. An entity may <a title="Entity Reference" href="#dt-entref">refer</a> to other entities to cause their inclusion in the document. A document begins in a "root" or <a title="Document Entity" href="#dt-docent">document entity</a>. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup. The logical and physical structures <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> nest properly, as described in <a href="#wf-entities"><b>4.3.2 Well-Formed Parsed Entities</b></a>.</p><div class="div2"> <h3><a name="sec-well-formed" id="sec-well-formed"/>2.1 Well-Formed XML Documents</h3><p> [<a name="dt-wellformed" id="dt-wellformed" title="Well-Formed">Definition</a>: A textual object is a <b>well-formed</b> XML document if:] </p><ol class="enumar"><li><p>Taken as a whole, it matches the production labeled <a href="#NT-document">document</a>.</p></li><li><p>It meets all the well-formedness constraints given in this specification.</p></li><li><p>Each of the <a title="Text Entity" href="#dt-parsedent">parsed entities</a> which is referenced directly or indirectly within the document is <a title="Well-Formed" href="#dt-wellformed">well-formed</a>.</p></li></ol> <h5><a name="document" id="document"/>Document</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-document" id="NT-document"/>[1] </td><td><code>document</code></td><td> ::= </td><td><code> <a href="#NT-prolog">prolog</a> <a href="#NT-element">element</a> <a href="#NT-Misc">Misc</a>*</code></td></tr></tbody></table><p>Matching the <a href="#NT-document">document</a> production implies that:</p><ol class="enumar"><li><p>It contains one or more <a title="Element" href="#dt-element">elements</a>.</p></li><li><p> [<a name="dt-root" id="dt-root" title="Root Element">Definition</a>: There is exactly one element, called the <b>root</b>, or document element, no part of which appears in the <a title="Content" href="#dt-content">content</a> of any other element.] For all other elements, if the <a title="Start-Tag" href="#dt-stag">start-tag</a> is in the content of another element, the <a title="End Tag" href="#dt-etag">end-tag</a> is in the content of the same element. More simply stated, the elements, delimited by start- and end-tags, nest properly within each other.</p></li></ol><p> [<a name="dt-parentchild" id="dt-parentchild" title="Parent/Child">Definition</a>: As a consequence of this, for each non-root element <code>C</code> in the document, there is one other element <code>P</code> in the document such that <code>C</code> is in the content of <code>P</code>, but is not in the content of any other element that is in the content of <code>P</code>. <code>P</code> is referred to as the <b>parent</b> of <code>C</code>, and <code>C</code> as a <b>child</b> of <code>P</code>.] </p></div><div class="div2"> <h3><a name="charsets" id="charsets"/>2.2 Characters</h3><p> [<a name="dt-text" id="dt-text" title="Text">Definition</a>: A parsed entity contains <b>text</b>, a sequence of <a title="Character" href="#dt-character">characters</a>, which may represent markup or character data.] [<a name="dt-character" id="dt-character" title="Character">Definition</a>: A <b>character</b> is an atomic unit of text as specified by ISO/IEC 10646:2000 <a href="#ISO10646">[ISO/IEC 10646]</a>. Legal characters are tab, carriage return, line feed, and the legal characters of Unicode and ISO/IEC 10646. The versions of these standards cited in <a href="#sec-existing-stds"><b>A.1 Normative References</b></a> were current at the time this document was prepared. New characters may be added to these standards by amendments or new editions. Consequently, XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> accept any character in the range specified for <a href="#NT-Char">Char</a>. ] </p> <h5><a name="char32" id="char32"/>Character Range</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-Char" id="NT-Char"/>[2] </td><td><code>Char</code></td><td> ::= </td><td><code>#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]</code></td><td><i>/* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */</i></td></tr></tbody></table><p>The mechanism for encoding character code points into bit patterns may vary from entity to entity. All XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> accept the UTF-8 and UTF-16 encodings of Unicode <span><a href="#Unicode">[Unicode]</a></span>; the mechanisms for signaling which of the two is in use, or for bringing other encodings into play, are discussed later, in <a href="#charencoding"><b>4.3.3 Character Encoding in Entities</b></a>.</p><div class="note"><p class="prefix"><b>Note:</b></p><p>Document authors are encouraged to avoid "compatibility characters", as defined in section <span>2.3</span> of <a href="#Unicode">[Unicode]</a>. The characters defined in the following ranges are also discouraged. They are either control characters or permanently undefined Unicode characters:</p><div class="exampleInner"><pre>[#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFD<span>E</span>F], [#x1FFFE-#x1FFFF], [#x2FFFE-#x2FFFF], [#x3FFFE-#x3FFFF], [#x4FFFE-#x4FFFF], [#x5FFFE-#x5FFFF], [#x6FFFE-#x6FFFF], [#x7FFFE-#x7FFFF], [#x8FFFE-#x8FFFF], [#x9FFFE-#x9FFFF], [#xAFFFE-#xAFFFF], [#xBFFFE-#xBFFFF], [#xCFFFE-#xCFFFF], [#xDFFFE-#xDFFFF], [#xEFFFE-#xEFFFF], [#xFFFFE-#xFFFFF], [#x10FFFE-#x10FFFF].</pre></div></div></div><div class="div2"> <h3><a name="sec-common-syn" id="sec-common-syn"/>2.3 Common Syntactic Constructs</h3><p>This section defines some symbols used widely in the grammar.</p><p> <a href="#NT-S">S</a> (white space) consists of one or more space (#x20) characters, carriage returns, line feeds, or tabs.</p> <h5><a name="white" id="white"/>White Space</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-S" id="NT-S"/>[3] </td><td><code>S</code></td><td> ::= </td><td><code>(#x20 | #x9 | #xD | #xA)+</code></td></tr></tbody></table><div class="note"><p class="prefix"><b>Note:</b></p><p>The presence of #xD in the above production is maintained purely for backward compatibility with the <a href="https://www.w3.org/TR/1998/REC-xml-19980210">First Edition</a>. As explained in <a href="#sec-line-ends"><b>2.11 End-of-Line Handling</b></a>, all #xD characters literally present in an XML document are either removed or replaced by #xA characters before any other processing is done. The only way to get a #xD character to match this production is to use a character reference in an entity value literal.</p></div><p>An <a href="#NT-Nmtoken">Nmtoken</a> (name token) is any mixture of name characters.</p><p>[<a name="dt-name" id="dt-name" title="Name">Definition</a>: A <a href="#NT-Name">Name</a> is an <a href="#NT-Nmtoken">Nmtoken</a> with a restricted set of initial characters.] Disallowed initial characters for <a href="#NT-Name">Names</a> include digits, diacritics, the full stop and the hyphen.</p><p>Names beginning with the string "<code>xml</code>", or with any string which would match <code>(('X'|'x') ('M'|'m') ('L'|'l'))</code>, are reserved for standardization in this or future versions of this specification.</p><div class="note"><p class="prefix"><b>Note:</b></p><p>The Namespaces in XML Recommendation <a href="#xml-names">[XML Names]</a> assigns a meaning to names containing colon characters. Therefore, authors should not use the colon in XML names except for namespace purposes, but XML processors must accept the colon as a name character.</p></div><p>The first character of a <a href="#NT-Name">Name</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be a <a href="#NT-NameStartChar">NameStartChar</a>, and any other characters <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be <a href="#NT-NameChar">NameChars</a>; this mechanism is used to prevent names from beginning with European (ASCII) digits or with basic combining characters. Almost all characters are permitted in names, except those which either are or reasonably could be used as delimiters. The intention is to be inclusive rather than exclusive, so that writing systems not yet encoded in Unicode can be used in XML names. See <a href="#sec-suggested-names"><b>J Suggestions for XML Names</b></a> for suggestions on the creation of names.</p><p>Document authors are encouraged to use names which are meaningful words or combinations of words in natural languages, and to avoid symbolic or white space characters in names. Note that COLON, HYPHEN-MINUS, FULL STOP (period), LOW LINE (underscore), and MIDDLE DOT are explicitly permitted.</p><p>The ASCII symbols and punctuation marks, along with a fairly large group of Unicode symbol characters, are excluded from names because they are more useful as delimiters in contexts where XML names are used outside XML documents; providing this group gives those contexts hard guarantees about what <em>cannot</em> be part of an XML name. The character #x037E, GREEK QUESTION MARK, is excluded because when normalized it becomes a semicolon, which could change the meaning of entity references.</p> <h5><a name="d0e804" id="d0e804"/>Names and Tokens</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-NameStartChar" id="NT-NameStartChar"/>[4] </td><td><code>NameStartChar</code></td><td> ::= </td><td><code>":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-NameChar" id="NT-NameChar"/>[4a] </td><td><code>NameChar</code></td><td> ::= </td><td><code><a href="#NT-NameStartChar">NameStartChar</a> | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-Name" id="NT-Name"/>[5] </td><td><code>Name</code></td><td> ::= </td><td><code><a href="#NT-NameStartChar">NameStartChar</a> (<a href="#NT-NameChar">NameChar</a>)*</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-Names" id="NT-Names"/>[6] </td><td><code>Names</code></td><td> ::= </td><td><code><a href="#NT-Name">Name</a> (#x20 <a href="#NT-Name">Name</a>)*</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-Nmtoken" id="NT-Nmtoken"/>[7] </td><td><code>Nmtoken</code></td><td> ::= </td><td><code>(<a href="#NT-NameChar">NameChar</a>)+</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-Nmtokens" id="NT-Nmtokens"/>[8] </td><td><code>Nmtokens</code></td><td> ::= </td><td><code><a href="#NT-Nmtoken">Nmtoken</a> (#x20 <a href="#NT-Nmtoken">Nmtoken</a>)*</code></td></tr></tbody></table><div class="note"><p class="prefix"><b>Note:</b></p><p>The <a href="#NT-Names">Names</a> and <a href="#NT-Nmtokens">Nmtokens</a> productions are used to define the validity of tokenized attribute values after normalization (see <a href="#sec-attribute-types"><b>3.3.1 Attribute Types</b></a>).</p></div><p>Literal data is any quoted string not containing the quotation mark used as a delimiter for that string. Literals are used for specifying the content of internal entities (<a href="#NT-EntityValue">EntityValue</a>), the values of attributes (<a href="#NT-AttValue">AttValue</a>), and external identifiers (<a href="#NT-SystemLiteral">SystemLiteral</a>). Note that a <a href="#NT-SystemLiteral">SystemLiteral</a> can be parsed without scanning for markup.</p> <h5><a name="d0e888" id="d0e888"/>Literals</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-EntityValue" id="NT-EntityValue"/>[9] </td><td><code>EntityValue</code></td><td> ::= </td><td><code>'"' ([^%&"] | <a href="#NT-PEReference">PEReference</a> | <a href="#NT-Reference">Reference</a>)* '"' </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| "'" ([^%&'] | <a href="#NT-PEReference">PEReference</a> | <a href="#NT-Reference">Reference</a>)* "'"</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-AttValue" id="NT-AttValue"/>[10] </td><td><code>AttValue</code></td><td> ::= </td><td><code>'"' ([^<&"] | <a href="#NT-Reference">Reference</a>)* '"' </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| "'" ([^<&'] | <a href="#NT-Reference">Reference</a>)* "'"</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-SystemLiteral" id="NT-SystemLiteral"/>[11] </td><td><code>SystemLiteral</code></td><td> ::= </td><td><code>('"' [^"]* '"') | ("'" [^']* "'") </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-PubidLiteral" id="NT-PubidLiteral"/>[12] </td><td><code>PubidLiteral</code></td><td> ::= </td><td><code>'"' <a href="#NT-PubidChar">PubidChar</a>* '"' | "'" (<a href="#NT-PubidChar">PubidChar</a> - "'")* "'"</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-PubidChar" id="NT-PubidChar"/>[13] </td><td><code>PubidChar</code></td><td> ::= </td><td><code>#x20 | #xD | #xA | [a-zA-Z0-9] | [-'()+,./:=?;!*#@$_%]</code></td></tr></tbody></table><div class="note"><p class="prefix"><b>Note:</b></p><p>Although the <a href="#NT-EntityValue">EntityValue</a> production allows the definition of a general entity consisting of a single explicit <code><</code> in the literal (e.g., <code><!ENTITY mylt "<"></code>), it is strongly advised to avoid this practice since any reference to that entity will cause a well-formedness error.</p></div></div><div class="div2"> <h3><a name="syntax" id="syntax"/>2.4 Character Data and Markup</h3><p> <a title="Text" href="#dt-text">Text</a> consists of intermingled <a title="Character Data" href="#dt-chardata">character data</a> and markup. [<a name="dt-markup" id="dt-markup" title="Markup">Definition</a>: <b>Markup</b> takes the form of <a title="Start-Tag" href="#dt-stag">start-tags</a>, <a title="End Tag" href="#dt-etag">end-tags</a>, <a title="Empty" href="#dt-empty">empty-element tags</a>, <a title="Entity Reference" href="#dt-entref">entity references</a>, <a title="Character Reference" href="#dt-charref">character references</a>, <a title="Comment" href="#dt-comment">comments</a>, <a title="CDATA Section" href="#dt-cdsection">CDATA section</a> delimiters, <a title="Document Type Declaration" href="#dt-doctype">document type declarations</a>, <a title="Processing instruction" href="#dt-pi">processing instructions</a>, <a href="#NT-XMLDecl">XML declarations</a>, <a href="#NT-TextDecl">text declarations</a>, and any white space that is at the top level of the document entity (that is, outside the document element and not inside any other markup).] </p><p> [<a name="dt-chardata" id="dt-chardata" title="Character Data">Definition</a>: All text that is not markup constitutes the <b>character data</b> of the document.] </p><p>The ampersand character (&) and the left angle bracket (<) <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear in their literal form, except when used as markup delimiters, or within a <a title="Comment" href="#dt-comment">comment</a>, a <a title="Processing instruction" href="#dt-pi">processing instruction</a>, or a <a title="CDATA Section" href="#dt-cdsection">CDATA section</a>. If they are needed elsewhere, they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be <a title="escape" href="#dt-escape">escaped</a> using either <a title="Character Reference" href="#dt-charref">numeric character references</a> or the strings " <code>&amp;</code> " and " <code>&lt;</code> " respectively. The right angle bracket (>) may be represented using the string " <code>&gt;</code> ", and <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em>, <a title="For Compatibility" href="#dt-compat">for compatibility</a>, be escaped using either " <code>&gt;</code> " or a character reference when it appears in the string " <code>]]></code> " in content, when that string is not marking the end of a <a title="CDATA Section" href="#dt-cdsection">CDATA section</a>.</p><p>In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup and does not include the CDATA-section-close delimiter, " <code>]]></code> ". In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, " <code>]]></code> ".</p><p>To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') may be represented as " <code>&apos;</code> ", and the double-quote character (") as " <code>&quot;</code> ".</p> <h5><a name="d0e1106" id="d0e1106"/>Character Data</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-CharData" id="NT-CharData"/>[14] </td><td><code>CharData</code></td><td> ::= </td><td><code>[^<&]* - ([^<&]* ']]>' [^<&]*)</code></td></tr></tbody></table></div><div class="div2"> <h3><a name="sec-comments" id="sec-comments"/>2.5 Comments</h3><p> [<a name="dt-comment" id="dt-comment" title="Comment">Definition</a>: <b>Comments</b> may appear anywhere in a document outside other <a title="Markup" href="#dt-markup">markup</a>; in addition, they may appear within the document type declaration at places allowed by the grammar. They are not part of the document's <a title="Character Data" href="#dt-chardata">character data</a>; an XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em>, but need not, make it possible for an application to retrieve the text of comments. <a title="For Compatibility" href="#dt-compat">For compatibility</a>, the string " <code>--</code> " (double-hyphen) <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> occur within comments.] Parameter entity references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be recognized within comments.</p> <h5><a name="d0e1149" id="d0e1149"/>Comments</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-Comment" id="NT-Comment"/>[15] </td><td><code>Comment</code></td><td> ::= </td><td><code>''</code></td></tr></tbody></table><p>An example of a comment:</p><div class="exampleInner"><pre></pre></div><p>Note that the grammar does not allow a comment ending in <code>---></code>. The following example is <em>not</em> well-formed.</p><div class="exampleInner"><pre></pre></div></div><div class="div2"> <h3><a name="sec-pi" id="sec-pi"/>2.6 Processing Instructions</h3><p> [<a name="dt-pi" id="dt-pi" title="Processing instruction">Definition</a>: <b>Processing instructions</b> (PIs) allow documents to contain instructions for applications.] </p> <h5><a name="d0e1188" id="d0e1188"/>Processing Instructions</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-PI" id="NT-PI"/>[16] </td><td><code>PI</code></td><td> ::= </td><td><code>'<?' <a href="#NT-PITarget">PITarget</a> (<a href="#NT-S">S</a> (<a href="#NT-Char">Char</a>* - (<a href="#NT-Char">Char</a>* '?>' <a href="#NT-Char">Char</a>*)))? '?>'</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-PITarget" id="NT-PITarget"/>[17] </td><td><code>PITarget</code></td><td> ::= </td><td><code> <a href="#NT-Name">Name</a> - (('X' | 'x') ('M' | 'm') ('L' | 'l'))</code></td></tr></tbody></table><p>PIs are not part of the document's <a title="Character Data" href="#dt-chardata">character data</a>, but <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be passed through to the application. The PI begins with a target (<a href="#NT-PITarget">PITarget</a>) used to identify the application to which the instruction is directed. The target names " <code>XML</code> ", " <code>xml</code> ", and so on are reserved for standardization in this or future versions of this specification. The XML <a title="Notation" href="#dt-notation">Notation</a> mechanism may be used for formal declaration of PI targets. Parameter entity references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be recognized within processing instructions.</p></div><div class="div2"> <h3><a name="sec-cdata-sect" id="sec-cdata-sect"/>2.7 CDATA Sections</h3><p> [<a name="dt-cdsection" id="dt-cdsection" title="CDATA Section">Definition</a>: <b>CDATA sections</b> may occur anywhere character data may occur; they are used to escape blocks of text containing characters which would otherwise be recognized as markup. CDATA sections begin with the string " <code><![CDATA[</code> " and end with the string " <code>]]></code> ":] </p> <h5><a name="d0e1271" id="d0e1271"/>CDATA Sections</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-CDSect" id="NT-CDSect"/>[18] </td><td><code>CDSect</code></td><td> ::= </td><td><code> <a href="#NT-CDStart">CDStart</a> <a href="#NT-CData">CData</a> <a href="#NT-CDEnd">CDEnd</a> </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-CDStart" id="NT-CDStart"/>[19] </td><td><code>CDStart</code></td><td> ::= </td><td><code>'<![CDATA['</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-CData" id="NT-CData"/>[20] </td><td><code>CData</code></td><td> ::= </td><td><code>(<a href="#NT-Char">Char</a>* - (<a href="#NT-Char">Char</a>* ']]>' <a href="#NT-Char">Char</a>*)) </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-CDEnd" id="NT-CDEnd"/>[21] </td><td><code>CDEnd</code></td><td> ::= </td><td><code>']]>'</code></td></tr></tbody></table><p>Within a CDATA section, only the <a href="#NT-CDEnd">CDEnd</a> string is recognized as markup, so that left angle brackets and ampersands may occur in their literal form; they need not (and cannot) be escaped using " <code>&lt;</code> " and " <code>&amp;</code> ". CDATA sections cannot nest.</p><p>An example of a CDATA section, in which " <code><greeting></code> " and " <code></greeting></code> " are recognized as <a title="Character Data" href="#dt-chardata">character data</a>, not <a title="Markup" href="#dt-markup">markup</a>:</p><div class="exampleInner"><pre><![CDATA[<greeting>Hello, world!</greeting>]]> </pre></div></div><div class="div2"> <h3><a name="sec-prolog-dtd" id="sec-prolog-dtd"/>2.8 Prolog and Document Type Declaration</h3><p> [<a name="dt-xmldecl" id="dt-xmldecl" title="XML Declaration">Definition</a>: XML documents <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> begin with an <b>XML declaration</b> which specifies the version of XML being used.] For example, the following is a complete XML document, <a title="Well-Formed" href="#dt-wellformed">well-formed</a> but not <a title="Validity" href="#dt-valid">valid</a>:</p><div class="exampleInner"><pre><?xml version="1.0"?> <greeting>Hello, world!</greeting> </pre></div><p>and so is this:</p><div class="exampleInner"><pre><greeting>Hello, world!</greeting></pre></div><p>The function of the markup in an XML document is to describe its storage and logical structure and to associate attribute name-value pairs with its logical structures. XML provides a mechanism, the <a title="Document Type Declaration" href="#dt-doctype">document type declaration</a>, to define constraints on the logical structure and to support the use of predefined storage units. [<a name="dt-valid" id="dt-valid" title="Validity">Definition</a>: An XML document is <b>valid</b> if it has an associated document type declaration and if the document complies with the constraints expressed in it.] </p><p>The document type declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> appear before the first <a title="Element" href="#dt-element">element</a> in the document.</p> <h5><a name="xmldoc" id="xmldoc"/>Prolog</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-prolog" id="NT-prolog"/>[22] </td><td><code>prolog</code></td><td> ::= </td><td><code> <a href="#NT-XMLDecl">XMLDecl</a>? <a href="#NT-Misc">Misc</a>* (<a href="#NT-doctypedecl">doctypedecl</a> <a href="#NT-Misc">Misc</a>*)?</code></td></tr><tr valign="baseline"><td><a name="NT-XMLDecl" id="NT-XMLDecl"/>[23] </td><td><code>XMLDecl</code></td><td> ::= </td><td><code>'<?xml' <a href="#NT-VersionInfo">VersionInfo</a> <a href="#NT-EncodingDecl">EncodingDecl</a>? <a href="#NT-SDDecl">SDDecl</a>? <a href="#NT-S">S</a>? '?>'</code></td></tr><tr valign="baseline"><td><a name="NT-VersionInfo" id="NT-VersionInfo"/>[24] </td><td><code>VersionInfo</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a> 'version' <a href="#NT-Eq">Eq</a> ("'" <a href="#NT-VersionNum">VersionNum</a> "'" | '"' <a href="#NT-VersionNum">VersionNum</a> '"')</code></td></tr><tr valign="baseline"><td><a name="NT-Eq" id="NT-Eq"/>[25] </td><td><code>Eq</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a>? '=' <a href="#NT-S">S</a>?</code></td></tr><tr valign="baseline"><td><a name="NT-VersionNum" id="NT-VersionNum"/>[26] </td><td><code>VersionNum</code></td><td> ::= </td><td><code>'1.' [0-9]+</code></td></tr><tr valign="baseline"><td><a name="NT-Misc" id="NT-Misc"/>[27] </td><td><code>Misc</code></td><td> ::= </td><td><code> <a href="#NT-Comment">Comment</a> | <a href="#NT-PI">PI</a> | <a href="#NT-S">S</a> </code></td></tr></tbody></table><p>Even though the <a href="#NT-VersionNum">VersionNum</a> production matches any version number of the form '1.x', XML 1.0 documents <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD NOT</em> specify a version number other than '1.0'.</p><div class="note"><p class="prefix"><b>Note:</b></p><p>When an XML 1.0 processor encounters a document that specifies a 1.x version number other than '1.0', it will process it as a 1.0 document. This means that an XML 1.0 processor will accept 1.x documents provided they do not use any non-1.0 features.</p></div><p> [<a name="dt-doctype" id="dt-doctype" title="Document Type Declaration">Definition</a>: The XML <b>document type declaration</b> contains or points to <a title="markup declaration" href="#dt-markupdecl">markup declarations</a> that provide a grammar for a class of documents. This grammar is known as a document type definition, or <b>DTD</b>. The document type declaration can point to an external subset (a special kind of <a title="External Entity" href="#dt-extent">external entity</a>) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.] </p><p> [<a name="dt-markupdecl" id="dt-markupdecl" title="markup declaration">Definition</a>: A <b>markup declaration</b> is an <a title="Element Type declaration" href="#dt-eldecl">element type declaration</a>, an <a title="Attribute-List Declaration" href="#dt-attdecl">attribute-list declaration</a>, an <a title="entity declaration" href="#dt-entdecl">entity declaration</a>, or a <a title="Notation Declaration" href="#dt-notdecl">notation declaration</a>.] These declarations may be contained in whole or in part within <a title="Parameter entity" href="#dt-PE">parameter entities</a>, as described in the well-formedness and validity constraints below. For further information, see <a href="#sec-physical-struct"><b>4 Physical Structures</b></a>.</p> <h5><a name="dtd" id="dtd"/>Document Type Definition</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-doctypedecl" id="NT-doctypedecl"/>[28] </td><td><code>doctypedecl</code></td><td> ::= </td><td><code>'<!DOCTYPE' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> (<a href="#NT-S">S</a> <a href="#NT-ExternalID">ExternalID</a>)? <a href="#NT-S">S</a>? ('[' <a href="#NT-intSubset">intSubset</a> ']' <a href="#NT-S">S</a>?)? '>'</code></td><td><a href="#vc-roottype">[VC: Root Element Type]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#ExtSubset">[WFC: External Subset]</a></td></tr><tr valign="baseline"><td><a name="NT-DeclSep" id="NT-DeclSep"/>[28a] </td><td><code>DeclSep</code></td><td> ::= </td><td><code> <a href="#NT-PEReference">PEReference</a> | <a href="#NT-S">S</a> </code></td><td><a href="#PE-between-Decls">[WFC: PE Between Declarations]</a></td></tr><tr valign="baseline"><td><a name="NT-intSubset" id="NT-intSubset"/>[28b] </td><td><code>intSubset</code></td><td> ::= </td><td><code>(<a href="#NT-markupdecl">markupdecl</a> | <a href="#NT-DeclSep">DeclSep</a>)*</code></td></tr><tr valign="baseline"><td><a name="NT-markupdecl" id="NT-markupdecl"/>[29] </td><td><code>markupdecl</code></td><td> ::= </td><td><code> <a href="#NT-elementdecl">elementdecl</a> | <a href="#NT-AttlistDecl">AttlistDecl</a> | <a href="#NT-EntityDecl">EntityDecl</a> | <a href="#NT-NotationDecl">NotationDecl</a> | <a href="#NT-PI">PI</a> | <a href="#NT-Comment">Comment</a> </code></td><td><a href="#vc-PEinMarkupDecl">[VC: Proper Declaration/PE Nesting]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#wfc-PEinInternalSubset">[WFC: PEs in Internal Subset]</a></td></tr></tbody></table><p>Note that it is possible to construct a well-formed document containing a <a href="#NT-doctypedecl">doctypedecl</a> that neither points to an external subset nor contains an internal subset.</p><p>The markup declarations may be made up in whole or in part of the <a title="Replacement Text" href="#dt-repltext">replacement text</a> of <a title="Parameter entity" href="#dt-PE">parameter entities</a>. The productions later in this specification for individual nonterminals (<a href="#NT-elementdecl">elementdecl</a>, <a href="#NT-AttlistDecl">AttlistDecl</a>, and so on) describe the declarations <em>after</em> all the parameter entities have been <a title="Include" href="#dt-include">included</a>.</p><p>Parameter entity references are recognized anywhere in the DTD (internal and external subsets and external parameter entities), except in literals, processing instructions, comments, and the contents of ignored conditional sections (see <a href="#sec-condition-sect"><b>3.4 Conditional Sections</b></a>). They are also recognized in entity value literals. The use of parameter entities in the internal subset is restricted as described below.</p><div class="constraint"><p class="prefix"><a name="vc-roottype" id="vc-roottype"/><b>Validity constraint: Root Element Type</b></p><p>The <a href="#NT-Name">Name</a> in the document type declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the element type of the <a title="Root Element" href="#dt-root">root element</a>.</p></div><div class="constraint"><p class="prefix"><a name="vc-PEinMarkupDecl" id="vc-PEinMarkupDecl"/><b>Validity constraint: Proper Declaration/PE Nesting</b></p><p>Parameter-entity <a title="Replacement Text" href="#dt-repltext">replacement text</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration (<a href="#NT-markupdecl">markupdecl</a> above) is contained in the replacement text for a <a title="Parameter-entity reference" href="#dt-PERef">parameter-entity reference</a>, both <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be contained in the same replacement text.</p></div><div class="constraint"><p class="prefix"><a name="wfc-PEinInternalSubset" id="wfc-PEinInternalSubset"/><b>Well-formedness constraint: PEs in Internal Subset</b></p><p>In the internal DTD subset, <a title="Parameter-entity reference" href="#dt-PERef">parameter-entity references</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> occur within markup declarations; they may occur where markup declarations can occur. (This does not apply to references that occur in external parameter entities or to the external subset.)</p></div><div class="constraint"><p class="prefix"><a name="ExtSubset" id="ExtSubset"/><b>Well-formedness constraint: External Subset</b></p><p>The external subset, if any, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the production for <a href="#NT-extSubset">extSubset</a>.</p></div><div class="constraint"><p class="prefix"><a name="PE-between-Decls" id="PE-between-Decls"/><b>Well-formedness constraint: PE Between Declarations</b></p><p>The replacement text of a parameter entity reference in a <a href="#NT-DeclSep">DeclSep</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the production <a href="#NT-extSubsetDecl">extSubsetDecl</a>.</p></div><p>Like the internal subset, the external subset and any external parameter entities referenced in a <a href="#NT-DeclSep">DeclSep</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> consist of a series of complete markup declarations of the types allowed by the non-terminal symbol <a href="#NT-markupdecl">markupdecl</a>, interspersed with white space or <a title="Parameter-entity reference" href="#dt-PERef">parameter-entity references</a>. However, portions of the contents of the external subset or of these external parameter entities may conditionally be ignored by using the <a title="conditional section" href="#dt-cond-section">conditional section</a> construct; this is not allowed in the internal subset but is allowed in external parameter entities referenced in the internal subset.</p> <h5><a name="ext-Subset" id="ext-Subset"/>External Subset</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-extSubset" id="NT-extSubset"/>[30] </td><td><code>extSubset</code></td><td> ::= </td><td><code> <a href="#NT-TextDecl">TextDecl</a>? <a href="#NT-extSubsetDecl">extSubsetDecl</a> </code></td></tr><tr valign="baseline"><td><a name="NT-extSubsetDecl" id="NT-extSubsetDecl"/>[31] </td><td><code>extSubsetDecl</code></td><td> ::= </td><td><code>( <a href="#NT-markupdecl">markupdecl</a> | <a href="#NT-conditionalSect">conditionalSect</a> | <a href="#NT-DeclSep">DeclSep</a>)*</code></td></tr></tbody></table><p>The external subset and external parameter entities also differ from the internal subset in that in them, <a title="Parameter-entity reference" href="#dt-PERef">parameter-entity references</a> are permitted <em>within</em> markup declarations, not only <em>between</em> markup declarations.</p><p>An example of an XML document with a document type declaration:</p><div class="exampleInner"><pre><?xml version="1.0"?> <!DOCTYPE greeting SYSTEM "hello.dtd"> <greeting>Hello, world!</greeting> </pre></div><p>The <a title="System Identifier" href="#dt-sysid">system identifier</a> " <code>hello.dtd</code> " gives the address (a URI reference) of a DTD for the document.</p><p>The declarations can also be given locally, as in this example:</p><div class="exampleInner"><pre><?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE greeting [ <!ELEMENT greeting (#PCDATA)> ]> <greeting>Hello, world!</greeting></pre></div><p>If both the external and internal subsets are used, the internal subset <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be considered to occur before the external subset. This has the effect that entity and attribute-list declarations in the internal subset take precedence over those in the external subset.</p></div><div class="div2"> <h3><a name="sec-rmd" id="sec-rmd"/>2.9 Standalone Document Declaration</h3><p>Markup declarations can affect the content of the document, as passed from an <a title="XML Processor" href="#dt-xml-proc">XML processor</a> to an application; examples are attribute defaults and entity declarations. The standalone document declaration, which may appear as a component of the XML declaration, signals whether or not there are such declarations which appear external to the <a title="Document Entity" href="#dt-docent">document entity</a> or in parameter entities. [<a name="dt-extmkpdecl" id="dt-extmkpdecl" title="External Markup Declaration">Definition</a>: An <b>external markup declaration</b> is defined as a markup declaration occurring in the external subset or in a parameter entity (external or internal, the latter being included because non-validating processors are not required to read them).] </p> <h5><a name="fulldtd" id="fulldtd"/>Standalone Document Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-SDDecl" id="NT-SDDecl"/>[32] </td><td><code>SDDecl</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a> 'standalone' <a href="#NT-Eq">Eq</a> (("'" ('yes' | 'no') "'") | ('"' ('yes' | 'no') '"')) </code></td><td><a href="#vc-check-rmd">[VC: Standalone Document Declaration]</a></td></tr></tbody></table><p>In a standalone document declaration, the value "yes" indicates that there are no <a title="External Markup Declaration" href="#dt-extmkpdecl">external markup declarations</a> which affect the information passed from the XML processor to the application. The value "no" indicates that there are or may be such external markup declarations. Note that the standalone document declaration only denotes the presence of external <em>declarations</em>; the presence, in a document, of references to external <em>entities</em>, when those entities are internally declared, does not change its standalone status.</p><p>If there are no external markup declarations, the standalone document declaration has no meaning. If there are external markup declarations but there is no standalone document declaration, the value "no" is assumed.</p><p>Any XML document for which <code>standalone="no"</code> holds can be converted algorithmically to a standalone document, which may be desirable for some network delivery applications.</p><div class="constraint"><p class="prefix"><a name="vc-check-rmd" id="vc-check-rmd"/><b>Validity constraint: Standalone Document Declaration</b></p><p>The standalone document declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> have the value "no" if any external markup declarations contain declarations of:</p><ul><li><p>attributes with <a title="Attribute Default" href="#dt-default">default</a> values, if elements to which these attributes apply appear in the document without specifications of values for these attributes, or</p></li><li><p>entities (other than <code>amp</code>, <code>lt</code>, <code>gt</code>, <code>apos</code>, <code>quot</code>), if <a title="Entity Reference" href="#dt-entref">references</a> to those entities appear in the document, or</p></li><li><p>attributes with tokenized types, where the attribute appears in the document with a value such that <a href="#AVNormalize"><cite>normalization</cite></a> will produce a different value from that which would be produced in the absence of the declaration, or</p></li><li><p>element types with <a title="Element content" href="#dt-elemcontent">element content</a>, if white space occurs directly within any instance of those types.</p></li></ul></div><p>An example XML declaration with a standalone document declaration:</p><div class="exampleInner"><pre><?xml version="1.0" standalone='yes'?></pre></div></div><div class="div2"> <h3><a name="sec-white-space" id="sec-white-space"/>2.10 White Space Handling</h3><p>In editing XML documents, it is often convenient to use "white space" (spaces, tabs, and blank lines) to set apart the markup for greater readability. Such white space is typically not intended for inclusion in the delivered version of the document. On the other hand, "significant" white space that should be preserved in the delivered version is common, for example in poetry and source code.</p><p>An <a title="XML Processor" href="#dt-xml-proc">XML processor</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> always pass all characters in a document that are not markup through to the application. A <a title="Validating Processor" href="#dt-validating"> validating XML processor</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> also inform the application which of these characters constitute white space appearing in <a title="Element content" href="#dt-elemcontent">element content</a>.</p><p>A special <a title="Attribute" href="#dt-attr">attribute</a> named <code>xml:space</code> may be attached to an element to signal an intention that in that element, white space should be preserved by applications. In valid documents, this attribute, like any other, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be <a title="Attribute-List Declaration" href="#dt-attdecl">declared</a> if it is used. When declared, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be given as an <a title="Enumerated Attribute Values" href="#dt-enumerated">enumerated type</a> whose values are one or both of "default" and "preserve". For example:</p><div class="exampleInner"><pre><!ATTLIST poem xml:space (default|preserve) 'preserve'> <!ATTLIST pre xml:space (preserve) #FIXED 'preserve'></pre></div><p>The value "default" signals that applications' default white-space processing modes are acceptable for this element; the value "preserve" indicates the intent that applications preserve all the white space. This declared intent is considered to apply to all elements within the content of the element where it is specified, unless overridden with another instance of the <code>xml:space</code> attribute. This specification does not give meaning to any value of <code>xml:space</code> other than "default" and "preserve". It is an error for other values to be specified; the XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> report the error or <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> recover by ignoring the attribute specification or by reporting the (erroneous) value to the application. Applications may ignore or reject erroneous values.</p><p>The <a title="Root Element" href="#dt-root">root element</a> of any document is considered to have signaled no intentions as regards application space handling, unless it provides a value for this attribute or the attribute is declared with a default value.</p></div><div class="div2"> <h3><a name="sec-line-ends" id="sec-line-ends"/>2.11 End-of-Line Handling</h3><p>XML <a title="Text Entity" href="#dt-parsedent">parsed entities</a> are often stored in computer files which, for editing convenience, are organized into lines. These lines are typically separated by some combination of the characters CARRIAGE RETURN (#xD) and LINE FEED (#xA).</p><p>To simplify the tasks of <a title="Application" href="#dt-app">applications</a>, the <a title="XML Processor" href="#dt-xml-proc">XML processor</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> behave as if it normalized all line breaks in external parsed entities (including the document entity) on input, before parsing, by translating both the two-character sequence #xD #xA and any #xD that is not followed by #xA to a single #xA character.</p></div><div class="div2"> <h3><a name="sec-lang-tag" id="sec-lang-tag"/>2.12 Language Identification</h3><p>In document processing, it is often useful to identify the natural or formal language in which the content is written. A special <a title="Attribute" href="#dt-attr">attribute</a> named <code>xml:lang</code> may be inserted in documents to specify the language used in the contents and attribute values of any element in an XML document. In valid documents, this attribute, like any other, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be <a title="Attribute-List Declaration" href="#dt-attdecl">declared</a> if it is used. The values of the attribute are language identifiers as defined by <a href="#RFC1766">[IETF BCP 47]</a>, <cite>Tags for the Identification of Languages</cite>; in addition, the empty string may be specified.</p><p>(Productions 33 through 38 have been removed.)</p><p>For example:</p><div class="exampleInner"><pre><p xml:lang="en">The quick brown fox jumps over the lazy dog.</p> <p xml:lang="en-GB">What colour is it?</p> <p xml:lang="en-US">What color is it?</p> <sp who="Faust" desc='leise' xml:lang="de"> <l>Habe nun, ach! Philosophie,</l> <l>Juristerei, und Medizin</l> <l>und leider auch Theologie</l> <l>durchaus studiert mit heißem Bemüh'n.</l> </sp></pre></div><p>The language specified by <code>xml:lang</code> applies to the element where it is specified (including the values of its attributes), and to all elements in its content unless overridden with another instance of <code>xml:lang</code>. In particular, the empty value of <code>xml:lang</code> is used on an element B to override a specification of <code>xml:lang</code> on an enclosing element A, without specifying another language. Within B, it is considered that there is no language information available, just as if <code>xml:lang</code> had not been specified on B or any of its ancestors. Applications determine which of an element's attribute values and which parts of its character content, if any, are treated as language-dependent values described by <code>xml:lang</code>. </p><div class="note"><p class="prefix"><b>Note:</b></p><p>Language information may also be provided by external transport protocols (e.g. HTTP or MIME). When available, this information may be used by XML applications, but the more local information provided by <code>xml:lang</code> should be considered to override it. </p></div><p>A simple declaration for <code>xml:lang</code> might take the form</p><div class="exampleInner"><pre>xml:lang CDATA #IMPLIED</pre></div><p>but specific default values may also be given, if appropriate. In a collection of French poems for English students, with glosses and notes in English, the <code>xml:lang</code> attribute might be declared this way:</p><div class="exampleInner"><pre><!ATTLIST poem xml:lang CDATA 'fr'> <!ATTLIST gloss xml:lang CDATA 'en'> <!ATTLIST note xml:lang CDATA 'en'></pre></div></div></div><div class="div1"> <h2><a name="sec-logical-struct" id="sec-logical-struct"/>3 Logical Structures</h2><p> [<a name="dt-element" id="dt-element" title="Element">Definition</a>: Each <a title="XML Document" href="#dt-xml-doc">XML document</a> contains one or more <b>elements</b>, the boundaries of which are either delimited by <a title="Start-Tag" href="#dt-stag">start-tags</a> and <a title="End Tag" href="#dt-etag">end-tags</a>, or, for <a title="Empty" href="#dt-empty">empty</a> elements, by an <a title="empty-element tag" href="#dt-eetag">empty-element tag</a>. Each element has a type, identified by name, sometimes called its "generic identifier" (GI), and may have a set of attribute specifications.] Each attribute specification has a <a title="Attribute Name" href="#dt-attrname">name</a> and a <a title="Attribute Value" href="#dt-attrval">value</a>.</p> <h5><a name="d0e2130" id="d0e2130"/>Element</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-element" id="NT-element"/>[39] </td><td><code>element</code></td><td> ::= </td><td><code> <a href="#NT-EmptyElemTag">EmptyElemTag</a> </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| <a href="#NT-STag">STag</a> <a href="#NT-content">content</a> <a href="#NT-ETag">ETag</a> </code></td><td><a href="#GIMatch">[WFC: Element Type Match]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#elementvalid">[VC: Element Valid]</a></td></tr></tbody></table><p>This specification does not constrain the application semantics, use, or (beyond syntax) names of the element types and attributes, except that names beginning with a match to <code>(('X'|'x')('M'|'m')('L'|'l'))</code> are reserved for standardization in this or future versions of this specification.</p><div class="constraint"><p class="prefix"><a name="GIMatch" id="GIMatch"/><b>Well-formedness constraint: Element Type Match</b></p><p>The <a href="#NT-Name">Name</a> in an element's end-tag <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the element type in the start-tag.</p></div><div class="constraint"><p class="prefix"><a name="elementvalid" id="elementvalid"/><b>Validity constraint: Element Valid</b></p><p>An element is valid if there is a declaration matching <a href="#NT-elementdecl">elementdecl</a> where the <a href="#NT-Name">Name</a> matches the element type, and one of the following holds:</p><ol class="enumar"><li><p>The declaration matches <b>EMPTY</b> and the element has no <a title="Content" href="#dt-content">content</a> (not even entity references, comments, PIs or white space).</p></li><li><p>The declaration matches <a href="#NT-children">children</a> and the sequence of <a title="Parent/Child" href="#dt-parentchild">child elements</a> belongs to the language generated by the regular expression in the content model, with optional white space, comments and PIs (i.e. markup matching production [27] <a href="#NT-Misc">Misc</a>) between the start-tag and the first child element, between child elements, or between the last child element and the end-tag. Note that a CDATA section containing only white space or a reference to an entity whose replacement text is character references expanding to white space do not match the nonterminal <a href="#NT-S">S</a>, and hence cannot appear in these positions; however, a reference to an internal entity with a literal value consisting of character references expanding to white space does match <a href="#NT-S">S</a>, since its replacement text is the white space resulting from expansion of the character references.</p></li><li><p>The declaration matches <a href="#NT-Mixed">Mixed</a>, and the content (after replacing any entity references with their replacement text) consists of <a title="Character Data" href="#dt-chardata">character data</a> (including <a title="CDATA Section" href="#dt-cdsection">CDATA sections</a>), <a title="Comment" href="#dt-comment">comments</a>, <a title="Processing instruction" href="#dt-pi">PIs</a> and <a title="Parent/Child" href="#dt-parentchild">child elements</a> whose types match names in the content model.</p></li><li><p>The declaration matches <b>ANY</b>, and the content (after replacing any entity references with their replacement text) consists of character data, <a title="CDATA Section" href="#dt-cdsection">CDATA sections</a>, <a title="Comment" href="#dt-comment">comments</a>, <a title="Processing instruction" href="#dt-pi">PIs</a> and <a title="Parent/Child" href="#dt-parentchild">child elements</a> whose types have been declared.</p></li></ol></div><div class="div2"> <h3><a name="sec-starttags" id="sec-starttags"/>3.1 Start-Tags, End-Tags, and Empty-Element Tags</h3><p> [<a name="dt-stag" id="dt-stag" title="Start-Tag">Definition</a>: The beginning of every non-empty XML element is marked by a <b>start-tag</b>.] </p> <h5><a name="d0e2259" id="d0e2259"/>Start-tag</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-STag" id="NT-STag"/>[40] </td><td><code>STag</code></td><td> ::= </td><td><code>'<' <a href="#NT-Name">Name</a> (<a href="#NT-S">S</a> <a href="#NT-Attribute">Attribute</a>)* <a href="#NT-S">S</a>? '>'</code></td><td><a href="#uniqattspec">[WFC: Unique Att Spec]</a></td></tr><tr valign="baseline"><td><a name="NT-Attribute" id="NT-Attribute"/>[41] </td><td><code>Attribute</code></td><td> ::= </td><td><code> <a href="#NT-Name">Name</a> <a href="#NT-Eq">Eq</a> <a href="#NT-AttValue">AttValue</a> </code></td><td><a href="#ValueType">[VC: Attribute Value Type]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#NoExternalRefs">[WFC: No External Entity References]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#CleanAttrVals">[WFC: No < in Attribute Values]</a></td></tr></tbody></table><p>The <a href="#NT-Name">Name</a> in the start- and end-tags gives the element's <b>type</b>. [<a name="dt-attr" id="dt-attr" title="Attribute">Definition</a>: The <a href="#NT-Name">Name</a>-<a href="#NT-AttValue">AttValue</a> pairs are referred to as the <b>attribute specifications</b> of the element], [<a name="dt-attrname" id="dt-attrname" title="Attribute Name">Definition</a>: with the <a href="#NT-Name">Name</a> in each pair referred to as the <b>attribute name</b> ] and [<a name="dt-attrval" id="dt-attrval" title="Attribute Value">Definition</a>: the content of the <a href="#NT-AttValue">AttValue</a> (the text between the <code>'</code> or <code>"</code> delimiters) as the <b>attribute value</b>.] Note that the order of attribute specifications in a start-tag or empty-element tag is not significant.</p><div class="constraint"><p class="prefix"><a name="uniqattspec" id="uniqattspec"/><b>Well-formedness constraint: Unique Att Spec</b></p><p>An attribute name <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear more than once in the same start-tag or empty-element tag.</p></div><div class="constraint"><p class="prefix"><a name="ValueType" id="ValueType"/><b>Validity constraint: Attribute Value Type</b></p><p>The attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> have been declared; the value <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be of the type declared for it. (For attribute types, see <a href="#attdecls"><b>3.3 Attribute-List Declarations</b></a>.)</p></div><div class="constraint"><p class="prefix"><a name="NoExternalRefs" id="NoExternalRefs"/><b>Well-formedness constraint: No External Entity References</b></p><p>Attribute values <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> contain direct or indirect entity references to external entities.</p></div><div class="constraint"><p class="prefix"><a name="CleanAttrVals" id="CleanAttrVals"/><b>Well-formedness constraint: No <code><</code> in Attribute Values</b></p><p>The <a title="Replacement Text" href="#dt-repltext">replacement text</a> of any entity referred to directly or indirectly in an attribute value <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> contain a <code><</code>.</p></div><p>An example of a start-tag:</p><div class="exampleInner"><pre><termdef id="dt-dog" term="dog"></pre></div><p> [<a name="dt-etag" id="dt-etag" title="End Tag">Definition</a>: The end of every element that begins with a start-tag <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be marked by an <b>end-tag</b> containing a name that echoes the element's type as given in the start-tag:] </p> <h5><a name="d0e2403" id="d0e2403"/>End-tag</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-ETag" id="NT-ETag"/>[42] </td><td><code>ETag</code></td><td> ::= </td><td><code>'</' <a href="#NT-Name">Name</a> <a href="#NT-S">S</a>? '>'</code></td></tr></tbody></table><p>An example of an end-tag:</p><div class="exampleInner"><pre></termdef></pre></div><p> [<a name="dt-content" id="dt-content" title="Content">Definition</a>: The <a title="Text" href="#dt-text">text</a> between the start-tag and end-tag is called the element's <b>content</b>:] </p> <h5><a name="d0e2433" id="d0e2433"/>Content of Elements</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-content" id="NT-content"/>[43] </td><td><code>content</code></td><td> ::= </td><td><code> <a href="#NT-CharData">CharData</a>? ((<a href="#NT-element">element</a> | <a href="#NT-Reference">Reference</a> | <a href="#NT-CDSect">CDSect</a> | <a href="#NT-PI">PI</a> | <a href="#NT-Comment">Comment</a>) <a href="#NT-CharData">CharData</a>?)*</code></td></tr></tbody></table><p> [<a name="dt-empty" id="dt-empty" title="Empty">Definition</a>: An element with no <a href="#NT-content">content</a> is said to be <b>empty</b>.] The representation of an empty element is either a start-tag immediately followed by an end-tag, or an empty-element tag. [<a name="dt-eetag" id="dt-eetag" title="empty-element tag">Definition</a>: An <b>empty-element tag</b> takes a special form:] </p> <h5><a name="d0e2480" id="d0e2480"/>Tags for Empty Elements</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-EmptyElemTag" id="NT-EmptyElemTag"/>[44] </td><td><code>EmptyElemTag</code></td><td> ::= </td><td><code>'<' <a href="#NT-Name">Name</a> (<a href="#NT-S">S</a> <a href="#NT-Attribute">Attribute</a>)* <a href="#NT-S">S</a>? '/>'</code></td><td><a href="#uniqattspec">[WFC: Unique Att Spec]</a></td></tr></tbody></table><p>Empty-element tags may be used for any element which has no content, whether or not it is declared using the keyword <b>EMPTY</b>. <a title="For interoperability" href="#dt-interop">For interoperability</a>, the empty-element tag <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be used, and <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> only be used, for elements which are declared EMPTY.</p><p>Examples of empty elements:</p><div class="exampleInner"><pre><IMG align="left" src="http://www.w3.org/Icons/WWW/w3c_home" /> <br></br> <br/></pre></div></div><div class="div2"> <h3><a name="elemdecls" id="elemdecls"/>3.2 Element Type Declarations</h3><p>The <a title="Element" href="#dt-element">element</a> structure of an <a title="XML Document" href="#dt-xml-doc">XML document</a> may, for <a title="Validity" href="#dt-valid">validation</a> purposes, be constrained using element type and attribute-list declarations. An element type declaration constrains the element's <a title="Content" href="#dt-content">content</a>.</p><p>Element type declarations often constrain which element types can appear as <a title="Parent/Child" href="#dt-parentchild">children</a> of the element. At user option, an XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> issue a warning when a declaration mentions an element type for which no declaration is provided, but this is not an error.</p><p> [<a name="dt-eldecl" id="dt-eldecl" title="Element Type declaration">Definition</a>: An <b>element type declaration</b> takes the form:] </p> <h5><a name="d0e2553" id="d0e2553"/>Element Type Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-elementdecl" id="NT-elementdecl"/>[45] </td><td><code>elementdecl</code></td><td> ::= </td><td><code>'<!ELEMENT' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-S">S</a> <a href="#NT-contentspec">contentspec</a> <a href="#NT-S">S</a>? '>'</code></td><td><a href="#EDUnique">[VC: Unique Element Type Declaration]</a></td></tr><tr valign="baseline"><td><a name="NT-contentspec" id="NT-contentspec"/>[46] </td><td><code>contentspec</code></td><td> ::= </td><td><code>'EMPTY' | 'ANY' | <a href="#NT-Mixed">Mixed</a> | <a href="#NT-children">children</a> </code></td></tr></tbody></table><p>where the <a href="#NT-Name">Name</a> gives the element type being declared.</p><div class="constraint"><p class="prefix"><a name="EDUnique" id="EDUnique"/><b>Validity constraint: Unique Element Type Declaration</b></p><p>An element type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be declared more than once.</p></div><p>Examples of element type declarations:</p><div class="exampleInner"><pre><!ELEMENT br EMPTY> <!ELEMENT p (#PCDATA|emph)* > <!ELEMENT %name.para; %content.para; > <!ELEMENT container ANY></pre></div><div class="div3"> <h4><a name="sec-element-content" id="sec-element-content"/>3.2.1 Element Content</h4><p> [<a name="dt-elemcontent" id="dt-elemcontent" title="Element content">Definition</a>: An element <a title="Start-Tag" href="#dt-stag">type</a> has <b>element content</b> when elements of that type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> contain only <a title="Parent/Child" href="#dt-parentchild">child</a> elements (no character data), optionally separated by white space (characters matching the nonterminal <a href="#NT-S">S</a>).] [<a name="dt-content-model" id="dt-content-model" title="Content model">Definition</a>: In this case, the constraint includes a <b>content model</b>, a simple grammar governing the allowed types of the child elements and the order in which they are allowed to appear.] The grammar is built on content particles (<a href="#NT-cp">cp</a>s), which consist of names, choice lists of content particles, or sequence lists of content particles:</p> <h5><a name="d0e2638" id="d0e2638"/>Element-content Models</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-children" id="NT-children"/>[47] </td><td><code>children</code></td><td> ::= </td><td><code>(<a href="#NT-choice">choice</a> | <a href="#NT-seq">seq</a>) ('?' | '*' | '+')?</code></td></tr><tr valign="baseline"><td><a name="NT-cp" id="NT-cp"/>[48] </td><td><code>cp</code></td><td> ::= </td><td><code>(<a href="#NT-Name">Name</a> | <a href="#NT-choice">choice</a> | <a href="#NT-seq">seq</a>) ('?' | '*' | '+')?</code></td></tr><tr valign="baseline"><td><a name="NT-choice" id="NT-choice"/>[49] </td><td><code>choice</code></td><td> ::= </td><td><code>'(' <a href="#NT-S">S</a>? <a href="#NT-cp">cp</a> ( <a href="#NT-S">S</a>? '|' <a href="#NT-S">S</a>? <a href="#NT-cp">cp</a> )+ <a href="#NT-S">S</a>? ')'</code></td><td><a href="#vc-PEinGroup">[VC: Proper Group/PE Nesting]</a></td></tr><tr valign="baseline"><td><a name="NT-seq" id="NT-seq"/>[50] </td><td><code>seq</code></td><td> ::= </td><td><code>'(' <a href="#NT-S">S</a>? <a href="#NT-cp">cp</a> ( <a href="#NT-S">S</a>? ',' <a href="#NT-S">S</a>? <a href="#NT-cp">cp</a> )* <a href="#NT-S">S</a>? ')'</code></td><td><a href="#vc-PEinGroup">[VC: Proper Group/PE Nesting]</a></td></tr></tbody></table><p>where each <a href="#NT-Name">Name</a> is the type of an element which may appear as a <a title="Parent/Child" href="#dt-parentchild">child</a>. Any content particle in a choice list may appear in the <a title="Element content" href="#dt-elemcontent">element content</a> at the location where the choice list appears in the grammar; content particles occurring in a sequence list <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> each appear in the <a title="Element content" href="#dt-elemcontent">element content</a> in the order given in the list. The optional character following a name or list governs whether the element or the content particles in the list may occur one or more (<code>+</code>), zero or more (<code>*</code>), or zero or one times (<code>?</code>). The absence of such an operator means that the element or content particle <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> appear exactly once. This syntax and meaning are identical to those used in the productions in this specification.</p><p>The content of an element matches a content model if and only if it is possible to trace out a path through the content model, obeying the sequence, choice, and repetition operators and matching each element in the content against an element type in the content model. <a title="For Compatibility" href="#dt-compat">For compatibility</a>, it is an error if the content model allows an element to match more than one occurrence of an element type in the content model. For more information, see <a href="#determinism"><b>E Deterministic Content Models</b></a>.</p><div class="constraint"><p class="prefix"><a name="vc-PEinGroup" id="vc-PEinGroup"/><b>Validity constraint: Proper Group/PE Nesting</b></p><p>Parameter-entity <a title="Replacement Text" href="#dt-repltext">replacement text</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be properly nested with parenthesized groups. That is to say, if either of the opening or closing parentheses in a <a href="#NT-choice">choice</a>, <a href="#NT-seq">seq</a>, or <a href="#NT-Mixed">Mixed</a> construct is contained in the replacement text for a <a title="Parameter-entity reference" href="#dt-PERef">parameter entity</a>, both <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be contained in the same replacement text.</p><p> <a title="For interoperability" href="#dt-interop">For interoperability</a>, if a parameter-entity reference appears in a <a href="#NT-choice">choice</a>, <a href="#NT-seq">seq</a>, or <a href="#NT-Mixed">Mixed</a> construct, its replacement text <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> contain at least one non-blank character, and neither the first nor last non-blank character of the replacement text <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be a connector (<code>|</code> or <code>,</code>).</p></div><p>Examples of element-content models:</p><div class="exampleInner"><pre><!ELEMENT spec (front, body, back?)> <!ELEMENT div1 (head, (p | list | note)*, div2*)> <!ELEMENT dictionary-body (%div.mix; | %dict.mix;)*></pre></div></div><div class="div3"> <h4><a name="sec-mixed-content" id="sec-mixed-content"/>3.2.2 Mixed Content</h4><p> [<a name="dt-mixed" id="dt-mixed" title="Mixed Content">Definition</a>: An element <a title="Start-Tag" href="#dt-stag">type</a> has <b>mixed content</b> when elements of that type may contain character data, optionally interspersed with <a title="Parent/Child" href="#dt-parentchild">child</a> elements.] In this case, the types of the child elements may be constrained, but not their order or their number of occurrences:</p> <h5><a name="d0e2826" id="d0e2826"/>Mixed-content Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-Mixed" id="NT-Mixed"/>[51] </td><td><code>Mixed</code></td><td> ::= </td><td><code>'(' <a href="#NT-S">S</a>? '#PCDATA' (<a href="#NT-S">S</a>? '|' <a href="#NT-S">S</a>? <a href="#NT-Name">Name</a>)* <a href="#NT-S">S</a>? ')*' </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| '(' <a href="#NT-S">S</a>? '#PCDATA' <a href="#NT-S">S</a>? ')' </code></td><td><a href="#vc-PEinGroup">[VC: Proper Group/PE Nesting]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#vc-MixedChildrenUnique">[VC: No Duplicate Types]</a></td></tr></tbody></table><p>where the <a href="#NT-Name">Name</a>s give the types of elements that may appear as children. The keyword <b>#PCDATA</b> derives historically from the term "parsed character data." </p><div class="constraint"><p class="prefix"><a name="vc-MixedChildrenUnique" id="vc-MixedChildrenUnique"/><b>Validity constraint: No Duplicate Types</b></p><p>The same name <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear more than once in a single mixed-content declaration.</p></div><p>Examples of mixed content declarations:</p><div class="exampleInner"><pre><!ELEMENT p (#PCDATA|a|ul|b|i|em)*> <!ELEMENT p (#PCDATA | %font; | %phrase; | %special; | %form;)* > <!ELEMENT b (#PCDATA)></pre></div></div></div><div class="div2"> <h3><a name="attdecls" id="attdecls"/>3.3 Attribute-List Declarations</h3><p> <a title="Attribute" href="#dt-attr">Attributes</a> are used to associate name-value pairs with <a title="Element" href="#dt-element">elements</a>. Attribute specifications <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear outside of <a title="Start-Tag" href="#dt-stag">start-tags</a> and <a title="empty-element tag" href="#dt-eetag">empty-element tags</a>; thus, the productions used to recognize them appear in <a href="#sec-starttags"><b>3.1 Start-Tags, End-Tags, and Empty-Element Tags</b></a>. Attribute-list declarations may be used:</p><ul><li><p>To define the set of attributes pertaining to a given element type.</p></li><li><p>To establish type constraints for these attributes.</p></li><li><p>To provide <a title="Attribute Default" href="#dt-default">default values</a> for attributes.</p></li></ul><p> [<a name="dt-attdecl" id="dt-attdecl" title="Attribute-List Declaration">Definition</a>: <b>Attribute-list declarations</b> specify the name, data type, and default value (if any) of each attribute associated with a given element type:] </p> <h5><a name="d0e2926" id="d0e2926"/>Attribute-list Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-AttlistDecl" id="NT-AttlistDecl"/>[52] </td><td><code>AttlistDecl</code></td><td> ::= </td><td><code>'<!ATTLIST' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-AttDef">AttDef</a>* <a href="#NT-S">S</a>? '>'</code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-AttDef" id="NT-AttDef"/>[53] </td><td><code>AttDef</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-S">S</a> <a href="#NT-AttType">AttType</a> <a href="#NT-S">S</a> <a href="#NT-DefaultDecl">DefaultDecl</a> </code></td></tr></tbody></table><p>The <a href="#NT-Name">Name</a> in the <a href="#NT-AttlistDecl">AttlistDecl</a> rule is the type of an element. At user option, an XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> issue a warning if attributes are declared for an element type not itself declared, but this is not an error. The <a href="#NT-Name">Name</a> in the <a href="#NT-AttDef">AttDef</a> rule is the name of the attribute.</p><p>When more than one <a href="#NT-AttlistDecl">AttlistDecl</a> is provided for a given element type, the contents of all those provided are merged. When more than one definition is provided for the same attribute of a given element type, the first declaration is binding and later declarations are ignored. <a title="For interoperability" href="#dt-interop">For interoperability,</a> writers of DTDs may choose to provide at most one attribute-list declaration for a given element type, at most one attribute definition for a given attribute name in an attribute-list declaration, and at least one attribute definition in each attribute-list declaration. For interoperability, an XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> at user option issue a warning when more than one attribute-list declaration is provided for a given element type, or more than one attribute definition is provided for a given attribute, but this is not an error.</p><div class="div3"> <h4><a name="sec-attribute-types" id="sec-attribute-types"/>3.3.1 Attribute Types</h4><p>XML attribute types are of three kinds: a string type, a set of tokenized types, and enumerated types. The string type may take any literal string as a value; the tokenized types are more constrained. The validity constraints noted in the grammar are applied after the attribute value has been normalized as described in <a href="#AVNormalize"><b>3.3.3 Attribute-Value Normalization</b></a>.</p> <h5><a name="d0e3004" id="d0e3004"/>Attribute Types</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-AttType" id="NT-AttType"/>[54] </td><td><code>AttType</code></td><td> ::= </td><td><code> <a href="#NT-StringType">StringType</a> | <a href="#NT-TokenizedType">TokenizedType</a> | <a href="#NT-EnumeratedType">EnumeratedType</a> </code></td></tr><tr valign="baseline"><td><a name="NT-StringType" id="NT-StringType"/>[55] </td><td><code>StringType</code></td><td> ::= </td><td><code>'CDATA'</code></td></tr><tr valign="baseline"><td><a name="NT-TokenizedType" id="NT-TokenizedType"/>[56] </td><td><code>TokenizedType</code></td><td> ::= </td><td><code>'ID'</code></td><td><a href="#id">[VC: ID]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#one-id-per-el">[VC: One ID per Element Type]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#id-default">[VC: ID Attribute Default]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'IDREF'</code></td><td><a href="#idref">[VC: IDREF]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'IDREFS'</code></td><td><a href="#idref">[VC: IDREF]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'ENTITY'</code></td><td><a href="#entname">[VC: Entity Name]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'ENTITIES'</code></td><td><a href="#entname">[VC: Entity Name]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'NMTOKEN'</code></td><td><a href="#nmtok">[VC: Name Token]</a></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'NMTOKENS'</code></td><td><a href="#nmtok">[VC: Name Token]</a></td></tr></tbody></table><div class="constraint"><p class="prefix"><a name="id" id="id"/><b>Validity constraint: ID</b></p><p>Values of type <b>ID</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the <a href="#NT-Name">Name</a> production. A name <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear more than once in an XML document as a value of this type; i.e., ID values <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> uniquely identify the elements which bear them.</p></div><div class="constraint"><p class="prefix"><a name="one-id-per-el" id="one-id-per-el"/><b>Validity constraint: One ID per Element Type</b></p><p>An element type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> have more than one ID attribute specified.</p></div><div class="constraint"><p class="prefix"><a name="id-default" id="id-default"/><b>Validity constraint: ID Attribute Default</b></p><p>An ID attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> have a declared default of <b>#IMPLIED</b> or <b>#REQUIRED</b>.</p></div><div class="constraint"><p class="prefix"><a name="idref" id="idref"/><b>Validity constraint: IDREF</b></p><p>Values of type <b>IDREF</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the <a href="#NT-Name">Name</a> production, and values of type <b>IDREFS</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match <a href="#NT-Names">Names</a>; each <a href="#NT-Name">Name</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the value of an ID attribute on some element in the XML document; i.e. <b>IDREF</b> values <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the value of some ID attribute.</p></div><div class="constraint"><p class="prefix"><a name="entname" id="entname"/><b>Validity constraint: Entity Name</b></p><p>Values of type <b>ENTITY</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the <a href="#NT-Name">Name</a> production, values of type <b>ENTITIES</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match <a href="#NT-Names">Names</a>; each <a href="#NT-Name">Name</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the name of an <a title="Unparsed Entity" href="#dt-unparsed">unparsed entity</a> declared in the <a title="Document Type Declaration" href="#dt-doctype">DTD</a>.</p></div><div class="constraint"><p class="prefix"><a name="nmtok" id="nmtok"/><b>Validity constraint: Name Token</b></p><p>Values of type <b>NMTOKEN</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the <a href="#NT-Nmtoken">Nmtoken</a> production; values of type <b>NMTOKENS</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match <a href="#NT-Nmtokens">Nmtokens</a>.</p></div><p> [<a name="dt-enumerated" id="dt-enumerated" title="Enumerated Attribute Values">Definition</a>: <b>Enumerated attributes</b> have a list of allowed values in their declaration ]. They <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> take one of those values. There are two kinds of enumerated attribute types:</p> <h5><a name="d0e3200" id="d0e3200"/>Enumerated Attribute Types</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-EnumeratedType" id="NT-EnumeratedType"/>[57] </td><td><code>EnumeratedType</code></td><td> ::= </td><td><code> <a href="#NT-NotationType">NotationType</a> | <a href="#NT-Enumeration">Enumeration</a> </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-NotationType" id="NT-NotationType"/>[58] </td><td><code>NotationType</code></td><td> ::= </td><td><code>'NOTATION' <a href="#NT-S">S</a> '(' <a href="#NT-S">S</a>? <a href="#NT-Name">Name</a> (<a href="#NT-S">S</a>? '|' <a href="#NT-S">S</a>? <a href="#NT-Name">Name</a>)* <a href="#NT-S">S</a>? ')' </code></td><td><a href="#notatn">[VC: Notation Attributes]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#OneNotationPer">[VC: One Notation Per Element Type]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#NoNotationEmpty">[VC: No Notation on Empty Element]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#NoDuplicateTokens">[VC: No Duplicate Tokens]</a></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-Enumeration" id="NT-Enumeration"/>[59] </td><td><code>Enumeration</code></td><td> ::= </td><td><code>'(' <a href="#NT-S">S</a>? <a href="#NT-Nmtoken">Nmtoken</a> (<a href="#NT-S">S</a>? '|' <a href="#NT-S">S</a>? <a href="#NT-Nmtoken">Nmtoken</a>)* <a href="#NT-S">S</a>? ')'</code></td><td><a href="#enum">[VC: Enumeration]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#NoDuplicateTokens">[VC: No Duplicate Tokens]</a></td></tr></tbody></table><p>A <b>NOTATION</b> attribute identifies a <a title="Notation" href="#dt-notation">notation</a>, declared in the DTD with associated system and/or public identifiers, to be used in interpreting the element to which the attribute is attached.</p><div class="constraint"><p class="prefix"><a name="notatn" id="notatn"/><b>Validity constraint: Notation Attributes</b></p><p>Values of this type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match one of the <a href="#Notations"><cite>notation</cite></a> names included in the declaration; all notation names in the declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be declared.</p></div><div class="constraint"><p class="prefix"><a name="OneNotationPer" id="OneNotationPer"/><b>Validity constraint: One Notation Per Element Type</b></p><p>An element type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> have more than one <b>NOTATION</b> attribute specified.</p></div><div class="constraint"><p class="prefix"><a name="NoNotationEmpty" id="NoNotationEmpty"/><b>Validity constraint: No Notation on Empty Element</b></p><p> <a title="For Compatibility" href="#dt-compat">For compatibility</a>, an attribute of type <b>NOTATION</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be declared on an element declared <b>EMPTY</b>.</p></div><div class="constraint"><p class="prefix"><a name="NoDuplicateTokens" id="NoDuplicateTokens"/><b>Validity constraint: No Duplicate Tokens</b></p><p>The notation names in a single <a href="#NT-NotationType">NotationType</a> attribute declaration, as well as the <a href="#NT-Nmtoken">NmToken</a>s in a single <a href="#NT-Enumeration">Enumeration</a> attribute declaration, <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> all be distinct.</p></div><div class="constraint"><p class="prefix"><a name="enum" id="enum"/><b>Validity constraint: Enumeration</b></p><p>Values of this type <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match one of the <a href="#NT-Nmtoken">Nmtoken</a> tokens in the declaration.</p></div><p> <a title="For interoperability" href="#dt-interop">For interoperability,</a> the same <a href="#NT-Nmtoken">Nmtoken</a> <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD NOT</em> occur more than once in the enumerated attribute types of a single element type.</p></div><div class="div3"> <h4><a name="sec-attr-defaults" id="sec-attr-defaults"/>3.3.2 Attribute Defaults</h4><p>An <a title="Attribute-List Declaration" href="#dt-attdecl">attribute declaration</a> provides information on whether the attribute's presence is <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em>, and if not, how an XML processor is to react if a declared attribute is absent in a document.</p> <h5><a name="d0e3369" id="d0e3369"/>Attribute Defaults</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-DefaultDecl" id="NT-DefaultDecl"/>[60] </td><td><code>DefaultDecl</code></td><td> ::= </td><td><code>'#REQUIRED' | '#IMPLIED' </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| (('#FIXED' <a href="#NT-S">S</a>)? <a href="#NT-AttValue">AttValue</a>)</code></td><td><a href="#RequiredAttr">[VC: Required Attribute]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#defattrvalid">[VC: Attribute Default Value Syntactically Correct]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#CleanAttrVals">[WFC: No < in Attribute Values]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#FixedAttr">[VC: Fixed Attribute Default]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#NoExternalRefs">[WFC: No External Entity References]</a></td></tr></tbody></table><p>In an attribute declaration, <b>#REQUIRED</b> means that the attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> always be provided, <b>#IMPLIED</b> that no default value is provided. [<a name="dt-default" id="dt-default" title="Attribute Default">Definition</a>: If the declaration is neither <b>#REQUIRED</b> nor <b>#IMPLIED</b>, then the <a href="#NT-AttValue">AttValue</a> value contains the declared <b>default</b> value; the <b>#FIXED</b> keyword states that the attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> always have the default value. When an XML processor encounters an element without a specification for an attribute for which it has read a default value declaration, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> report the attribute with the declared default value to the application.] </p><div class="constraint"><p class="prefix"><a name="RequiredAttr" id="RequiredAttr"/><b>Validity constraint: Required Attribute</b></p><p>If the default declaration is the keyword <b>#REQUIRED</b>, then the attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be specified for all elements of the type in the attribute-list declaration.</p></div><div class="constraint"><p class="prefix"><a name="defattrvalid" id="defattrvalid"/><b>Validity constraint: Attribute Default Value Syntactically Correct</b></p><p>The declared default value <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> meet the syntactic constraints of the declared attribute type. That is, the default value of an attribute: </p><ul><li><p> of type IDREF or ENTITY must match the <a href="#NT-Name">Name</a> production;</p></li><li><p> of type IDREFS or ENTITIES must match the <a href="#NT-Names">Names</a> production;</p></li><li><p> of type NMTOKEN must match the <a href="#NT-Nmtoken">Nmtoken</a> production;</p></li><li><p> of type NMTOKENS must match the <a href="#NT-Nmtokens">Nmtokens</a> production;</p></li><li><p> of an <a href="#NT-EnumeratedType">enumerated type</a> (either a <a href="#NT-NotationType">NOTATION</a> type or an <a href="#NT-Enumeration">enumeration</a>) must match one of the enumerated values.</p></li></ul><p>Note that only the syntactic constraints of the type are required here; other constraints (e.g. that the value be the name of a declared unparsed entity, for an attribute of type ENTITY) will be reported by a validating parser only if an element without a specification for this attribute actually occurs.</p></div><div class="constraint"><p class="prefix"><a name="FixedAttr" id="FixedAttr"/><b>Validity constraint: Fixed Attribute Default</b></p><p>If an attribute has a default value declared with the <b>#FIXED</b> keyword, instances of that attribute <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the default value.</p></div><p>Examples of attribute-list declarations:</p><div class="exampleInner"><pre><!ATTLIST termdef id ID #REQUIRED name CDATA #IMPLIED> <!ATTLIST list type (bullets|ordered|glossary) "ordered"> <!ATTLIST form method CDATA #FIXED "POST"></pre></div></div><div class="div3"> <h4><a name="AVNormalize" id="AVNormalize"/>3.3.3 Attribute-Value Normalization</h4><p>Before the value of an attribute is passed to the application or checked for validity, the XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> normalize the attribute value by applying the algorithm below, or by using some other method such that the value passed to the application is the same as that produced by the algorithm.</p><ol class="enumar"><li><p>All line breaks <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> have been normalized on input to #xA as described in <a href="#sec-line-ends"><b>2.11 End-of-Line Handling</b></a>, so the rest of this algorithm operates on text normalized in this way.</p></li><li><p>Begin with a normalized value consisting of the empty string.</p></li><li><p>For each character, entity reference, or character reference in the unnormalized attribute value, beginning with the first and continuing to the last, do the following:</p><ul><li><p>For a character reference, append the referenced character to the normalized value.</p></li><li><p>For an entity reference, recursively apply step 3 of this algorithm to the replacement text of the entity.</p></li><li><p>For a white space character (#x20, #xD, #xA, #x9), append a space character (#x20) to the normalized value.</p></li><li><p>For another character, append the character to the normalized value.</p></li></ul></li></ol><p>If the attribute type is not CDATA, then the XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> further process the normalized attribute value by discarding any leading and trailing space (#x20) characters, and by replacing sequences of space (#x20) characters by a single space (#x20) character.</p><p>Note that if the unnormalized attribute value contains a character reference to a white space character other than space (#x20), the normalized value contains the referenced character itself (#xD, #xA or #x9). This contrasts with the case where the unnormalized value contains a white space character (not a reference), which is replaced with a space character (#x20) in the normalized value and also contrasts with the case where the unnormalized value contains an entity reference whose replacement text contains a white space character; being recursively processed, the white space character is replaced with a space character (#x20) in the normalized value.</p><p>All attributes for which no declaration has been read <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be treated by a non-validating processor as if declared <b>CDATA</b>.</p><p>It is an error if an <a title="Attribute Value" href="#dt-attrval">attribute value</a> contains a <a title="Entity Reference" href="#dt-entref">reference</a> to an entity for which no declaration has been read.</p><p>Following are examples of attribute normalization. Given the following declarations:</p><div class="exampleInner"><pre><!ENTITY d "&#xD;"> <!ENTITY a "&#xA;"> <!ENTITY da "&#xD;&#xA;"></pre></div><p>the attribute specifications in the left column below would be normalized to the character sequences of the middle column if the attribute <code>a</code> is declared <b>NMTOKENS</b> and to those of the right columns if <code>a</code> is declared <b>CDATA</b>.</p><table border="1" frame="border"><thead><tr><th>Attribute specification</th><th>a is NMTOKENS</th><th>a is CDATA</th></tr></thead><tbody><tr><td> <div class="exampleInner"><pre>a=" xyz"</pre></div> </td><td> <div class="exampleInner"><pre>x y z</pre></div> </td><td> <div class="exampleInner"><pre>#x20 #x20 x y z</pre></div> </td></tr><tr><td> <div class="exampleInner"><pre>a="&d;&d;A&a;&#x20;&a;B&da;"</pre></div> </td><td> <div class="exampleInner"><pre>A #x20 B</pre></div> </td><td> <div class="exampleInner"><pre>#x20 #x20 A #x20 #x20 #x20 B #x20 #x20</pre></div> </td></tr><tr><td> <div class="exampleInner"><pre>a= "&#xd;&#xd;A&#xa;&#xa;B&#xd;&#xa;"</pre></div> </td><td> <div class="exampleInner"><pre>#xD #xD A #xA #xA B #xD #xA</pre></div> </td><td> <div class="exampleInner"><pre>#xD #xD A #xA #xA B #xD #xA</pre></div> </td></tr></tbody></table><p>Note that the last example is invalid (but well-formed) if <code>a</code> is declared to be of type <b>NMTOKENS</b>.</p></div></div><div class="div2"> <h3><a name="sec-condition-sect" id="sec-condition-sect"/>3.4 Conditional Sections</h3><p> [<a name="dt-cond-section" id="dt-cond-section" title="conditional section">Definition</a>: <b>Conditional sections</b> are portions of the <a title="Document Type Declaration" href="#dt-doctype">document type declaration external subset</a> or of external parameter entities which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.] </p> <h5><a name="d0e3658" id="d0e3658"/>Conditional Section</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-conditionalSect" id="NT-conditionalSect"/>[61] </td><td><code>conditionalSect</code></td><td> ::= </td><td><code> <a href="#NT-includeSect">includeSect</a> | <a href="#NT-ignoreSect">ignoreSect</a> </code></td></tr><tr valign="baseline"><td><a name="NT-includeSect" id="NT-includeSect"/>[62] </td><td><code>includeSect</code></td><td> ::= </td><td><code>'<![' <a href="#NT-S">S</a>? 'INCLUDE' <a href="#NT-S">S</a>? '[' <a href="#NT-extSubsetDecl">extSubsetDecl</a> ']]>' </code></td><td><a href="#condsec-nesting">[VC: Proper Conditional Section/PE Nesting]</a></td></tr><tr valign="baseline"><td><a name="NT-ignoreSect" id="NT-ignoreSect"/>[63] </td><td><code>ignoreSect</code></td><td> ::= </td><td><code>'<![' <a href="#NT-S">S</a>? 'IGNORE' <a href="#NT-S">S</a>? '[' <a href="#NT-ignoreSectContents">ignoreSectContents</a>* ']]>'</code></td><td><a href="#condsec-nesting">[VC: Proper Conditional Section/PE Nesting]</a></td></tr><tr valign="baseline"><td><a name="NT-ignoreSectContents" id="NT-ignoreSectContents"/>[64] </td><td><code>ignoreSectContents</code></td><td> ::= </td><td><code> <a href="#NT-Ignore">Ignore</a> ('<![' <a href="#NT-ignoreSectContents">ignoreSectContents</a> ']]>' <a href="#NT-Ignore">Ignore</a>)*</code></td></tr><tr valign="baseline"><td><a name="NT-Ignore" id="NT-Ignore"/>[65] </td><td><code>Ignore</code></td><td> ::= </td><td><code> <a href="#NT-Char">Char</a>* - (<a href="#NT-Char">Char</a>* ('<![' | ']]>') <a href="#NT-Char">Char</a>*) </code></td></tr></tbody></table><div class="constraint"><p class="prefix"><a name="condsec-nesting" id="condsec-nesting"/><b>Validity constraint: Proper Conditional Section/PE Nesting</b></p><p>If any of the "<code><![</code>", "<code>[</code>", or "<code>]]></code>" of a conditional section is contained in the replacement text for a parameter-entity reference, all of them <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be contained in the same replacement text.</p></div><p>Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, processing instructions, or nested conditional sections, intermingled with white space.</p><p>If the keyword of the conditional section is <b>INCLUDE</b>, then the contents of the conditional section <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be processed as part of the DTD. If the keyword of the conditional section is <b>IGNORE</b>, then the contents of the conditional section <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> <em class="rfc2119" title="Keyword in RFC 2119 context">NOT</em> be processed as part of the DTD. If a conditional section with a keyword of <b>INCLUDE</b> occurs within a larger conditional section with a keyword of <b>IGNORE</b>, both the outer and the inner conditional sections <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be ignored. The contents of an ignored conditional section <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be parsed by ignoring all characters after the "<code>[</code>" following the keyword, except conditional section starts "<code><![</code>" and ends "<code>]]></code>", until the matching conditional section end is found. Parameter entity references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be recognized in this process.</p><p>If the keyword of the conditional section is a parameter-entity reference, the parameter entity <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be replaced by its content before the processor decides whether to include or ignore the conditional section.</p><p>An example:</p><div class="exampleInner"><pre><!ENTITY % draft 'INCLUDE' > <!ENTITY % final 'IGNORE' > <![%draft;[ <!ELEMENT book (comments*, title, body, supplements?)> ]]> <![%final;[ <!ELEMENT book (title, body, supplements?)> ]]></pre></div></div></div><div class="div1"> <h2><a name="sec-physical-struct" id="sec-physical-struct"/>4 Physical Structures</h2><p> [<a name="dt-entity" id="dt-entity" title="Entity">Definition</a>: An XML document may consist of one or many storage units. These are called <b>entities</b>; they all have <b>content</b> and are all (except for the <a title="Document Entity" href="#dt-docent">document entity</a> and the <a title="Document Type Declaration" href="#dt-doctype">external DTD subset</a>) identified by entity <b>name</b>.] Each XML document has one entity called the <a title="Document Entity" href="#dt-docent">document entity</a>, which serves as the starting point for the <a title="XML Processor" href="#dt-xml-proc">XML processor</a> and may contain the whole document.</p><p>Entities may be either parsed or unparsed. [<a name="dt-parsedent" id="dt-parsedent" title="Text Entity">Definition</a>: The contents of a <b>parsed entity</b> are referred to as its <a title="Replacement Text" href="#dt-repltext">replacement text</a>; this <a title="Text" href="#dt-text">text</a> is considered an integral part of the document.] </p><p> [<a name="dt-unparsed" id="dt-unparsed" title="Unparsed Entity">Definition</a>: An <b>unparsed entity</b> is a resource whose contents may or may not be <a title="Text" href="#dt-text">text</a>, and if text, may be other than XML. Each unparsed entity has an associated <a title="Notation" href="#dt-notation">notation</a>, identified by name. Beyond a requirement that an XML processor make the identifiers for the entity and notation available to the application, XML places no constraints on the contents of unparsed entities.] </p><p>Parsed entities are invoked by name using entity references; unparsed entities by name, given in the value of <b>ENTITY</b> or <b>ENTITIES</b> attributes.</p><p> [<a name="gen-entity" id="gen-entity" title="general entity">Definition</a>: <b>General entities</b> are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term <em>entity</em> when this leads to no ambiguity.] [<a name="dt-PE" id="dt-PE" title="Parameter entity">Definition</a>: <b>Parameter entities</b> are parsed entities for use within the DTD.] These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.</p><div class="div2"> <h3><a name="sec-references" id="sec-references"/>4.1 Character and Entity References</h3><p> [<a name="dt-charref" id="dt-charref" title="Character Reference">Definition</a>: A <b>character reference</b> refers to a specific character in the ISO/IEC 10646 character set, for example one not directly accessible from available input devices.] </p> <h5><a name="d0e3895" id="d0e3895"/>Character Reference</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-CharRef" id="NT-CharRef"/>[66] </td><td><code>CharRef</code></td><td> ::= </td><td><code>'&#' [0-9]+ ';' </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| '&#x' [0-9a-fA-F]+ ';'</code></td><td><a href="#wf-Legalchar">[WFC: Legal Character]</a></td></tr></tbody></table><div class="constraint"><p class="prefix"><a name="wf-Legalchar" id="wf-Legalchar"/><b>Well-formedness constraint: Legal Character</b></p><p>Characters referred to using character references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the production for <a href="#NT-Char">Char</a>.</p></div><p>If the character reference begins with " <code>&#x</code> ", the digits and letters up to the terminating <code>;</code> provide a hexadecimal representation of the character's code point in ISO/IEC 10646. If it begins just with " <code>&#</code> ", the digits up to the terminating <code>;</code> provide a decimal representation of the character's code point.</p><p> [<a name="dt-entref" id="dt-entref" title="Entity Reference">Definition</a>: An <b>entity reference</b> refers to the content of a named entity.] [<a name="dt-GERef" id="dt-GERef" title="General Entity Reference">Definition</a>: References to parsed general entities use ampersand (<code>&</code>) and semicolon (<code>;</code>) as delimiters.] [<a name="dt-PERef" id="dt-PERef" title="Parameter-entity reference">Definition</a>: <b>Parameter-entity references</b> use percent-sign (<code>%</code>) and semicolon (<code>;</code>) as delimiters.] </p> <h5><a name="d0e3966" id="d0e3966"/>Entity Reference</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-Reference" id="NT-Reference"/>[67] </td><td><code>Reference</code></td><td> ::= </td><td><code> <a href="#NT-EntityRef">EntityRef</a> | <a href="#NT-CharRef">CharRef</a> </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-EntityRef" id="NT-EntityRef"/>[68] </td><td><code>EntityRef</code></td><td> ::= </td><td><code>'&' <a href="#NT-Name">Name</a> ';'</code></td><td><a href="#wf-entdeclared">[WFC: Entity Declared]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#vc-entdeclared">[VC: Entity Declared]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#textent">[WFC: Parsed Entity]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#norecursion">[WFC: No Recursion]</a></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-PEReference" id="NT-PEReference"/>[69] </td><td><code>PEReference</code></td><td> ::= </td><td><code>'%' <a href="#NT-Name">Name</a> ';'</code></td><td><a href="#vc-entdeclared">[VC: Entity Declared]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#norecursion">[WFC: No Recursion]</a></td></tr><tr valign="baseline"><td/><td/><td/><td/><td><a href="#indtd">[WFC: In DTD]</a></td></tr></tbody></table><div class="constraint"><p class="prefix"><a name="wf-entdeclared" id="wf-entdeclared"/><b>Well-formedness constraint: Entity Declared</b></p><p>In a document without any DTD, a document with only an internal DTD subset which contains no parameter entity references, or a document with " <code>standalone='yes'</code> ", for an entity reference that does not occur within the external subset or a parameter entity, the <a href="#NT-Name">Name</a> given in the entity reference <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> <a title="match" href="#dt-match">match</a> that in an <a href="#sec-entity-decl"><cite>entity declaration</cite></a> that does not occur within the external subset or a parameter entity, except that well-formed documents need not declare any of the following entities: <code>amp</code>, <code>lt</code>, <code>gt</code>, <code>apos</code>, <code>quot</code>. The declaration of a general entity <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> precede any reference to it which appears in a default value in an attribute-list declaration.</p><p>Note that non-validating processors are <a href="#include-if-valid"><cite>not obligated to</cite></a> read and process entity declarations occurring in parameter entities or in the external subset; for such documents, the rule that an entity must be declared is a well-formedness constraint only if <a href="#sec-rmd"><cite>standalone='yes'</cite></a>.</p></div><div class="constraint"><p class="prefix"><a name="vc-entdeclared" id="vc-entdeclared"/><b>Validity constraint: Entity Declared</b></p><p>In a document with an external subset or parameter entity references<span>, if the document is not standalone (either "<code>standalone='no'</code>" is specified or there is no standalone declaration), then</span> the <a href="#NT-Name">Name</a> given in the entity reference <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> <a title="match" href="#dt-match">match</a> that in an <a href="#sec-entity-decl"><cite>entity declaration</cite></a>. For interoperability, valid documents <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> declare the entities <code>amp</code>, <code>lt</code>, <code>gt</code>, <code>apos</code>, <code>quot</code>, in the form specified in <a href="#sec-predefined-ent"><b>4.6 Predefined Entities</b></a>. The declaration of a parameter entity <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> precede any reference to it. Similarly, the declaration of a general entity <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> precede any attribute-list declaration containing a default value with a direct or indirect reference to that general entity.</p></div><div class="constraint"><p class="prefix"><a name="textent" id="textent"/><b>Well-formedness constraint: Parsed Entity</b></p><p>An entity reference <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> contain the name of an <a title="Unparsed Entity" href="#dt-unparsed">unparsed entity</a>. Unparsed entities may be referred to only in <a title="Attribute Value" href="#dt-attrval">attribute values</a> declared to be of type <b>ENTITY</b> or <b>ENTITIES</b>.</p></div><div class="constraint"><p class="prefix"><a name="norecursion" id="norecursion"/><b>Well-formedness constraint: No Recursion</b></p><p>A parsed entity <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> contain a recursive reference to itself, either directly or indirectly.</p></div><div class="constraint"><p class="prefix"><a name="indtd" id="indtd"/><b>Well-formedness constraint: In DTD</b></p><p>Parameter-entity references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear outside the <a title="Document Type Declaration" href="#dt-doctype">DTD</a>.</p></div><p>Examples of character and entity references:</p><div class="exampleInner"><pre>Type <key>less-than</key> (&#x3C;) to save options. This document was prepared on &docdate; and is classified &security-level;.</pre></div><p>Example of a parameter-entity reference:</p><div class="exampleInner"><pre> <!ENTITY % ISOLat2 SYSTEM "http://www.xml.com/iso/isolat2-xml.entities" >  %ISOLat2;</pre></div></div><div class="div2"> <h3><a name="sec-entity-decl" id="sec-entity-decl"/>4.2 Entity Declarations</h3><p> [<a name="dt-entdecl" id="dt-entdecl" title="entity declaration">Definition</a>: Entities are declared thus:] </p> <h5><a name="d0e4163" id="d0e4163"/>Entity Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-EntityDecl" id="NT-EntityDecl"/>[70] </td><td><code>EntityDecl</code></td><td> ::= </td><td><code> <a href="#NT-GEDecl">GEDecl</a> | <a href="#NT-PEDecl">PEDecl</a> </code></td></tr><tr valign="baseline"><td><a name="NT-GEDecl" id="NT-GEDecl"/>[71] </td><td><code>GEDecl</code></td><td> ::= </td><td><code>'<!ENTITY' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-S">S</a> <a href="#NT-EntityDef">EntityDef</a> <a href="#NT-S">S</a>? '>'</code></td></tr><tr valign="baseline"><td><a name="NT-PEDecl" id="NT-PEDecl"/>[72] </td><td><code>PEDecl</code></td><td> ::= </td><td><code>'<!ENTITY' <a href="#NT-S">S</a> '%' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-S">S</a> <a href="#NT-PEDef">PEDef</a> <a href="#NT-S">S</a>? '>'</code></td></tr><tr valign="baseline"><td><a name="NT-EntityDef" id="NT-EntityDef"/>[73] </td><td><code>EntityDef</code></td><td> ::= </td><td><code> <a href="#NT-EntityValue">EntityValue</a> | (<a href="#NT-ExternalID">ExternalID</a> <a href="#NT-NDataDecl">NDataDecl</a>?)</code></td></tr><tr valign="baseline"><td><a name="NT-PEDef" id="NT-PEDef"/>[74] </td><td><code>PEDef</code></td><td> ::= </td><td><code> <a href="#NT-EntityValue">EntityValue</a> | <a href="#NT-ExternalID">ExternalID</a> </code></td></tr></tbody></table><p>The <a href="#NT-Name">Name</a> identifies the entity in an <a title="Entity Reference" href="#dt-entref">entity reference</a> or, in the case of an unparsed entity, in the value of an <b>ENTITY</b> or <b>ENTITIES</b> attribute. If the same entity is declared more than once, the first declaration encountered is binding; at user option, an XML processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> issue a warning if entities are declared multiple times.</p><div class="div3"> <h4><a name="sec-internal-ent" id="sec-internal-ent"/>4.2.1 Internal Entities</h4><p> [<a name="dt-internent" id="dt-internent" title="Internal Entity Replacement Text">Definition</a>: If the entity definition is an <a href="#NT-EntityValue">EntityValue</a>, the defined entity is called an <b>internal entity</b>. There is no separate physical storage object, and the content of the entity is given in the declaration.] Note that some processing of entity and character references in the <a title="Literal Entity Value" href="#dt-litentval">literal entity value</a> may be required to produce the correct <a title="Replacement Text" href="#dt-repltext">replacement text</a>: see <a href="#intern-replacement"><b>4.5 Construction of Entity Replacement Text</b></a>.</p><p>An internal entity is a <a title="Text Entity" href="#dt-parsedent">parsed entity</a>.</p><p>Example of an internal entity declaration:</p><div class="exampleInner"><pre><!ENTITY Pub-Status "This is a pre-release of the specification."></pre></div></div><div class="div3"> <h4><a name="sec-external-ent" id="sec-external-ent"/>4.2.2 External Entities</h4><p> [<a name="dt-extent" id="dt-extent" title="External Entity">Definition</a>: If the entity is not internal, it is an <b>external entity</b>, declared as follows:] </p> <h5><a name="d0e4313" id="d0e4313"/>External Entity Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-ExternalID" id="NT-ExternalID"/>[75] </td><td><code>ExternalID</code></td><td> ::= </td><td><code>'SYSTEM' <a href="#NT-S">S</a> <a href="#NT-SystemLiteral">SystemLiteral</a> </code></td></tr><tr valign="baseline"><td/><td/><td/><td><code>| 'PUBLIC' <a href="#NT-S">S</a> <a href="#NT-PubidLiteral">PubidLiteral</a> <a href="#NT-S">S</a> <a href="#NT-SystemLiteral">SystemLiteral</a> </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-NDataDecl" id="NT-NDataDecl"/>[76] </td><td><code>NDataDecl</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a> 'NDATA' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> </code></td><td><a href="#not-declared">[VC: Notation Declared]</a></td></tr></tbody></table><p>If the <a href="#NT-NDataDecl">NDataDecl</a> is present, this is a general <a title="Unparsed Entity" href="#dt-unparsed">unparsed entity</a>; otherwise it is a parsed entity.</p><div class="constraint"><p class="prefix"><a name="not-declared" id="not-declared"/><b>Validity constraint: Notation Declared</b></p><p>The <a href="#NT-Name">Name</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> match the declared name of a <a title="Notation" href="#dt-notation">notation</a>.</p></div><p> [<a name="dt-sysid" id="dt-sysid" title="System Identifier">Definition</a>: The <a href="#NT-SystemLiteral">SystemLiteral</a> is called the entity's <b>system identifier</b>. It is meant to be converted to a URI reference (as defined in <a href="#rfc3986">[IETF RFC 3986]</a>), as part of the process of dereferencing it to obtain input for the XML processor to construct the entity's replacement text.] It is an error for a fragment identifier (beginning with a <code>#</code> character) to be part of a system identifier. Unless otherwise provided by information outside the scope of this specification (e.g. a special XML element type defined by a particular DTD, or a processing instruction defined by a particular application specification), relative URIs are relative to the location of the resource within which the entity declaration occurs. This is defined to be the external entity containing the '<' which starts the declaration, at the point when it is parsed as a declaration. A URI might thus be relative to the <a title="Document Entity" href="#dt-docent">document entity</a>, to the entity containing the <a title="Document Type Declaration" href="#dt-doctype">external DTD subset</a>, or to some other <a title="External Entity" href="#dt-extent">external parameter entity</a>. Attempts to retrieve the resource identified by a URI may be redirected at the parser level (for example, in an entity resolver) or below (at the protocol level, for example, via an HTTP <code>Location:</code> header). In the absence of additional information outside the scope of this specification within the resource, the base URI of a resource is always the URI of the actual resource returned. In other words, it is the URI of the resource retrieved after all redirection has occurred.</p><p>System identifiers (and other XML strings meant to be used as URI references) may contain characters that, according to <a href="#rfc3986">[IETF RFC 3986]</a>, must be escaped before a URI can be used to retrieve the referenced resource. The characters to be escaped are the control characters #x0 to #x1F and #x7F (most of which cannot appear in XML), space #x20, the delimiters '<' #x3C, '>' #x3E and '"' #x22, the <em>unwise</em> characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C, '^' #x5E and '`' #x60, as well as all characters above #x7F. Since escaping is not always a fully reversible process, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be performed only when absolutely necessary and as late as possible in a processing chain. In particular, neither the process of converting a relative URI to an absolute one nor the process of passing a URI reference to a process or software component responsible for dereferencing it <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> trigger escaping. When escaping does occur, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be performed as follows:</p><ol class="enumar"><li><p>Each character to be escaped is represented in UTF-8 <span><a href="#Unicode">[Unicode]</a></span> as one or more bytes.</p></li><li><p>The resulting bytes are escaped with the URI escaping mechanism (that is, converted to <code>%</code> <var>HH</var>, where HH is the hexadecimal notation of the byte value).</p></li><li><p>The original character is replaced by the resulting character sequence.</p></li></ol><div class="note"><p class="prefix"><b>Note:</b></p><p>In a future edition of this specification, the XML Core Working Group intends to replace the preceding paragraph and list of steps with a normative reference to an upcoming revision of IETF RFC 3987, which will define "Legacy Extended IRIs (LEIRIs)". When this revision is available, it is the intent of the XML Core WG to use it to replace language similar to the above in any future revisions of XML-related specifications under its purview.</p></div><p> [<a name="dt-pubid" id="dt-pubid" title="Public identifier">Definition</a>: In addition to a system identifier, an external identifier may include a <b>public identifier</b>.] An XML processor attempting to retrieve the entity's content may use any combination of the public and system identifiers as well as additional information outside the scope of this specification to try to generate an alternative URI reference. If the processor is unable to do so, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> use the URI reference specified in the system literal. Before a match is attempted, all strings of white space in the public identifier <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be normalized to single space characters (#x20), and leading and trailing white space <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be removed.</p><p>Examples of external entity declarations:</p><div class="exampleInner"><pre><!ENTITY open-hatch SYSTEM "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY open-hatch PUBLIC "-//Textuality//TEXT Standard open-hatch boilerplate//EN" "http://www.textuality.com/boilerplate/OpenHatch.xml"> <!ENTITY hatch-pic SYSTEM "../grafix/OpenHatch.gif" NDATA gif ></pre></div></div></div><div class="div2"> <h3><a name="TextEntities" id="TextEntities"/>4.3 Parsed Entities</h3><div class="div3"> <h4><a name="sec-TextDecl" id="sec-TextDecl"/>4.3.1 The Text Declaration</h4><p>External parsed entities <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> each begin with a <b>text declaration</b>.</p> <h5><a name="d0e4481" id="d0e4481"/>Text Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-TextDecl" id="NT-TextDecl"/>[77] </td><td><code>TextDecl</code></td><td> ::= </td><td><code>'<?xml' <a href="#NT-VersionInfo">VersionInfo</a>? <a href="#NT-EncodingDecl">EncodingDecl</a> <a href="#NT-S">S</a>? '?>'</code></td></tr></tbody></table><p>The text declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be provided literally, not by reference to a parsed entity. The text declaration <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> appear at any position other than the beginning of an external parsed entity. The text declaration in an external parsed entity is not considered part of its <a title="Replacement Text" href="#dt-repltext">replacement text</a>.</p></div><div class="div3"> <h4><a name="wf-entities" id="wf-entities"/>4.3.2 Well-Formed Parsed Entities</h4><p>The document entity is well-formed if it matches the production labeled <a href="#NT-document">document</a>. An external general parsed entity is well-formed if it matches the production labeled <a href="#NT-extParsedEnt">extParsedEnt</a>. All external parameter entities are well-formed by definition.</p><div class="note"><p class="prefix"><b>Note:</b></p><p>Only parsed entities that are referenced directly or indirectly within the document are required to be well-formed.</p></div> <h5><a name="d0e4524" id="d0e4524"/>Well-Formed External Parsed Entity</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-extParsedEnt" id="NT-extParsedEnt"/>[78] </td><td><code>extParsedEnt</code></td><td> ::= </td><td><code> <a href="#NT-TextDecl">TextDecl</a>? <a href="#NT-content">content</a> </code></td></tr></tbody></table><p>An internal general parsed entity is well-formed if its replacement text matches the production labeled <a href="#NT-content">content</a>. All internal parameter entities are well-formed by definition.</p><p>A consequence of well-formedness in general entities is that the logical and physical structures in an XML document are properly nested; no <a title="Start-Tag" href="#dt-stag">start-tag</a>, <a title="End Tag" href="#dt-etag">end-tag</a>, <a title="Empty" href="#dt-empty">empty-element tag</a>, <a title="Element" href="#dt-element">element</a>, <a title="Comment" href="#dt-comment">comment</a>, <a title="Processing instruction" href="#dt-pi">processing instruction</a>, <a title="Character Reference" href="#dt-charref">character reference</a>, or <a title="Entity Reference" href="#dt-entref">entity reference</a> can begin in one entity and end in another.</p></div><div class="div3"> <h4><a name="charencoding" id="charencoding"/>4.3.3 Character Encoding in Entities</h4><p>Each external parsed entity in an XML document may use a different encoding for its characters. All XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be able to read entities in both the UTF-8 and UTF-16 encodings. The terms "UTF-8" and "UTF-16" in this specification do not apply to <span>related character encodings, including but not limited to UTF-16BE, UTF-16LE, or CESU-8.</span></p><p>Entities encoded in UTF-16 <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> and entities encoded in UTF-8 <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> begin with the Byte Order Mark described by Annex H of <a href="#ISO10646-2000">[ISO/IEC 10646:2000]</a>, section <span>16.8</span> of <a href="#Unicode">[Unicode]</a> (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.</p><p>If the replacement text of an external entity is to begin with the character U+FEFF, and no text declaration is present, then a Byte Order Mark MUST be present, whether the entity is encoded in UTF-8 or UTF-16.</p><p>Although an XML processor is required to read only entities in the UTF-8 and UTF-16 encodings, it is recognized that other encodings are used around the world, and it may be desired for XML processors to read entities that use them. In the absence of external character encoding information (such as MIME headers), parsed entities which are stored in an encoding other than UTF-8 or UTF-16 <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> begin with a text declaration (see <a href="#sec-TextDecl"><b>4.3.1 The Text Declaration</b></a>) containing an encoding declaration:</p> <h5><a name="d0e4623" id="d0e4623"/>Encoding Declaration</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-EncodingDecl" id="NT-EncodingDecl"/>[80] </td><td><code>EncodingDecl</code></td><td> ::= </td><td><code> <a href="#NT-S">S</a> 'encoding' <a href="#NT-Eq">Eq</a> ('"' <a href="#NT-EncName">EncName</a> '"' | "'" <a href="#NT-EncName">EncName</a> "'" ) </code></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-EncName" id="NT-EncName"/>[81] </td><td><code>EncName</code></td><td> ::= </td><td><code>[A-Za-z] ([A-Za-z0-9._] | '-')*</code></td><td><i>/* Encoding name contains only Latin characters */</i></td></tr></tbody></table><p>In the <a title="Document Entity" href="#dt-docent">document entity</a>, the encoding declaration is part of the <a title="XML Declaration" href="#dt-xmldecl">XML declaration</a>. The <a href="#NT-EncName">EncName</a> is the name of the encoding used.</p><p>In an encoding declaration, the values " <code>UTF-8</code> ", " <code>UTF-16</code> ", " <code>ISO-10646-UCS-2</code> ", and " <code>ISO-10646-UCS-4</code> " <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be used for the various encodings and transformations of Unicode / ISO/IEC 10646, the values " <code>ISO-8859-1</code> ", " <code>ISO-8859-2</code> ", ... " <code>ISO-8859-</code> <var>n</var> " (where <var>n</var> is the part number) <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be used for the parts of ISO 8859, and the values " <code>ISO-2022-JP</code> ", " <code>Shift_JIS</code> ", and " <code>EUC-JP</code> " <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> be used for the various encoded forms of JIS X-0208-1997. It is <em class="rfc2119" title="Keyword in RFC 2119 context">RECOMMENDED</em> that character encodings registered (as <em>charset</em>s) with the Internet Assigned Numbers Authority <a href="#IANA">[IANA-CHARSETS]</a>, other than those just listed, be referred to using their registered names; other encodings <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> use names starting with an "x-" prefix. XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> match character encoding names in a case-insensitive way and <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> either interpret an IANA-registered name as the encoding registered at IANA for that name or treat it as unknown (processors are, of course, not required to support all IANA-registered encodings).</p><p>In the absence of information provided by an external transport protocol (e.g. HTTP or MIME), it is a <a title="Fatal Error" href="#dt-fatal">fatal error</a> for an entity including an encoding declaration to be presented to the XML processor in an encoding other than that named in the declaration, or for an entity which begins with neither a Byte Order Mark nor an encoding declaration to use an encoding other than UTF-8. Note that since ASCII is a subset of UTF-8, ordinary ASCII entities do not strictly need an encoding declaration.</p><p>It is a <a title="Fatal Error" href="#dt-fatal">fatal error</a> for a <a href="#NT-TextDecl">TextDecl</a> to occur other than at the beginning of an external entity.</p><p>It is a <a title="Fatal Error" href="#dt-fatal">fatal error</a> when an XML processor encounters an entity with an encoding that it is unable to process. It is a <a title="Fatal Error" href="#dt-fatal">fatal error</a> if an XML entity is determined (via default, encoding declaration, or higher-level protocol) to be in a certain encoding but contains byte sequences that are not legal in that encoding. Specifically, it is a fatal error if an entity encoded in UTF-8 contains any <span>ill-formed</span> code unit sequences, as defined in <span>section 3.9 of </span> Unicode <span><a href="#Unicode">[Unicode]</a></span>. Unless an encoding is determined by a higher-level protocol, it is also a <a title="Fatal Error" href="#dt-fatal">fatal error</a> if an XML entity contains no encoding declaration and its content is not legal UTF-8 or UTF-16.</p><p>Examples of text declarations containing encoding declarations:</p><div class="exampleInner"><pre><?xml encoding='UTF-8'?> <?xml encoding='EUC-JP'?></pre></div></div></div><div class="div2"> <h3><a name="entproc" id="entproc"/>4.4 XML Processor Treatment of Entities and References</h3><p>The table below summarizes the contexts in which character references, entity references, and invocations of unparsed entities might appear and the <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em> behavior of an <a title="XML Processor" href="#dt-xml-proc">XML processor</a> in each case. The labels in the leftmost column describe the recognition context: </p><dl><dt class="label">Reference in Content</dt><dd><p>as a reference anywhere after the <a title="Start-Tag" href="#dt-stag">start-tag</a> and before the <a title="End Tag" href="#dt-etag">end-tag</a> of an element; corresponds to the nonterminal <a href="#NT-content">content</a>.</p></dd><dt class="label">Reference in Attribute Value</dt><dd><p>as a reference within either the value of an attribute in a <a title="Start-Tag" href="#dt-stag">start-tag</a>, or a default value in an <a title="Attribute-List Declaration" href="#dt-attdecl">attribute declaration</a>; corresponds to the nonterminal <a href="#NT-AttValue">AttValue</a>.</p></dd><dt class="label">Occurs as Attribute Value</dt><dd><p>as a <a href="#NT-Name">Name</a>, not a reference, appearing either as the value of an attribute which has been declared as type <b>ENTITY</b>, or as one of the space-separated tokens in the value of an attribute which has been declared as type <b>ENTITIES</b>.</p></dd><dt class="label">Reference in Entity Value</dt><dd><p>as a reference within a parameter or internal entity's <a title="Literal Entity Value" href="#dt-litentval">literal entity value</a> in the entity's declaration; corresponds to the nonterminal <a href="#NT-EntityValue">EntityValue</a>.</p></dd><dt class="label">Reference in DTD</dt><dd><p>as a reference within either the internal or external subsets of the <a title="Document Type Declaration" href="#dt-doctype">DTD</a>, but outside of an <a href="#NT-EntityValue">EntityValue</a>, <a href="#NT-AttValue">AttValue</a>, <a href="#NT-PI">PI</a>, <a href="#NT-Comment">Comment</a>, <a href="#NT-SystemLiteral">SystemLiteral</a>, <a href="#NT-PubidLiteral">PubidLiteral</a>, or the contents of an ignored conditional section (see <a href="#sec-condition-sect"><b>3.4 Conditional Sections</b></a>).</p><p>.</p></dd></dl><p> </p><table border="1" frame="border" cellpadding="7"><tbody align="center"><tr><td rowspan="2"/><td colspan="4" align="center" valign="bottom">Entity Type</td><td rowspan="2" align="center">Character</td></tr><tr align="center" valign="bottom"><td>Parameter</td><td>Internal General</td><td>External Parsed General</td><td>Unparsed</td></tr><tr align="center" valign="middle"><td align="right">Reference in Content</td><td> <a href="#not-recognized"><cite>Not recognized</cite></a> </td><td> <a href="#included"><cite>Included</cite></a> </td><td> <a href="#include-if-valid"><cite>Included if validating</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#included"><cite>Included</cite></a> </td></tr><tr align="center" valign="middle"><td align="right">Reference in Attribute Value</td><td> <a href="#not-recognized"><cite>Not recognized</cite></a> </td><td> <a href="#inliteral"><cite>Included in literal</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#included"><cite>Included</cite></a> </td></tr><tr align="center" valign="middle"><td align="right">Occurs as Attribute Value</td><td> <a href="#not-recognized"><cite>Not recognized</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#notify"><cite>Notify</cite></a> </td><td> <a href="#not-recognized"><cite>Not recognized</cite></a> </td></tr><tr align="center" valign="middle"><td align="right">Reference in EntityValue</td><td> <a href="#inliteral"><cite>Included in literal</cite></a> </td><td> <a href="#bypass"><cite>Bypassed</cite></a> </td><td> <a href="#bypass"><cite>Bypassed</cite></a> </td><td> <a href="#error"><cite>Error</cite></a> </td><td> <a href="#included"><cite>Included</cite></a> </td></tr><tr align="center" valign="middle"><td align="right">Reference in DTD</td><td> <a href="#as-PE"><cite>Included as PE</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td><td> <a href="#forbidden"><cite>Forbidden</cite></a> </td></tr></tbody></table><div class="div3"> <h4><a name="not-recognized" id="not-recognized"/>4.4.1 Not Recognized</h4><p>Outside the DTD, the <code>%</code> character has no special significance; thus, what would be parameter entity references in the DTD are not recognized as markup in <a href="#NT-content">content</a>. Similarly, the names of unparsed entities are not recognized except when they appear in the value of an appropriately declared attribute.</p></div><div class="div3"> <h4><a name="included" id="included"/>4.4.2 Included</h4><p> [<a name="dt-include" id="dt-include" title="Include">Definition</a>: An entity is <b>included</b> when its <a title="Replacement Text" href="#dt-repltext">replacement text</a> is retrieved and processed, in place of the reference itself, as though it were part of the document at the location the reference was recognized.] The replacement text may contain both <a title="Character Data" href="#dt-chardata">character data</a> and (except for parameter entities) <a title="Markup" href="#dt-markup">markup</a>, which <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be recognized in the usual way. (The string " <code>AT&amp;T;</code> " expands to " <code>AT&T;</code> " and the remaining ampersand is not recognized as an entity-reference delimiter.) A character reference is <b>included</b> when the indicated character is processed in place of the reference itself. </p></div><div class="div3"> <h4><a name="include-if-valid" id="include-if-valid"/>4.4.3 Included If Validating</h4><p>When an XML processor recognizes a reference to a parsed entity, in order to <a title="Validity" href="#dt-valid">validate</a> the document, the processor <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> <a title="Include" href="#dt-include">include</a> its replacement text. If the entity is external, and the processor is not attempting to validate the XML document, the processor <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em>, but need not, include the entity's replacement text. If a non-validating processor does not include the replacement text, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> inform the application that it recognized, but did not read, the entity.</p><p>This rule is based on the recognition that the automatic inclusion provided by the SGML and XML entity mechanism, primarily designed to support modularity in authoring, is not necessarily appropriate for other applications, in particular document browsing. Browsers, for example, when encountering an external parsed entity reference, might choose to provide a visual indication of the entity's presence and retrieve it for display only on demand.</p></div><div class="div3"> <h4><a name="forbidden" id="forbidden"/>4.4.4 Forbidden</h4><p>The following are forbidden, and constitute <a title="Fatal Error" href="#dt-fatal">fatal errors</a>:</p><ul><li><p>the appearance of a reference to an <a title="Unparsed Entity" href="#dt-unparsed">unparsed entity</a>, except in the <a href="#NT-EntityValue">EntityValue</a> in an entity declaration.</p></li><li><p>the appearance of any character or general-entity reference in the DTD except within an <a href="#NT-EntityValue">EntityValue</a> or <a href="#NT-AttValue">AttValue</a>.</p></li><li><p>a reference to an external entity in an attribute value.</p></li></ul></div><div class="div3"> <h4><a name="inliteral" id="inliteral"/>4.4.5 Included in Literal</h4><p>When an <a title="Entity Reference" href="#dt-entref">entity reference</a> appears in an attribute value, or a parameter entity reference appears in a literal entity value, its <a title="Replacement Text" href="#dt-repltext">replacement text</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be processed in place of the reference itself as though it were part of the document at the location the reference was recognized, except that a single or double quote character in the replacement text <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> always be treated as a normal data character and <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> terminate the literal. For example, this is well-formed:</p><div class="exampleInner"><pre><!ENTITY % YN '"Yes"' > <!ENTITY WhatHeSaid "He said %YN;" ></pre></div><p>while this is not:</p><div class="exampleInner"><pre><!ENTITY EndAttr "27'" > <element attribute='a-&EndAttr;></pre></div></div><div class="div3"> <h4><a name="notify" id="notify"/>4.4.6 Notify</h4><p>When the name of an <a title="Unparsed Entity" href="#dt-unparsed">unparsed entity</a> appears as a token in the value of an attribute of declared type <b>ENTITY</b> or <b>ENTITIES</b>, a validating processor <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> inform the application of the <a title="System Identifier" href="#dt-sysid">system</a> and <a title="Public identifier" href="#dt-pubid">public</a> (if any) identifiers for both the entity and its associated <a title="Notation" href="#dt-notation">notation</a>.</p></div><div class="div3"> <h4><a name="bypass" id="bypass"/>4.4.7 Bypassed</h4><p>When a general entity reference appears in the <a href="#NT-EntityValue">EntityValue</a> in an entity declaration, it <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be bypassed and left as is.</p></div><div class="div3"> <h4><a name="as-PE" id="as-PE"/>4.4.8 Included as PE</h4><p>Just as with external parsed entities, parameter entities need only be <a href="#include-if-valid"><cite>included if validating</cite></a>. When a parameter-entity reference is recognized in the DTD and included, its <a title="Replacement Text" href="#dt-repltext">replacement text</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be enlarged by the attachment of one leading and one following space (#x20) character; the intent is to constrain the replacement text of parameter entities to contain an integral number of grammatical tokens in the DTD. This behavior <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> apply to parameter entity references within entity values; these are described in <a href="#inliteral"><b>4.4.5 Included in Literal</b></a>.</p></div><div class="div3"> <h4><a name="error" id="error"/>4.4.9 Error</h4><p>It is an <a title="Error" href="#dt-error">error</a> for a reference to an unparsed entity to appear in the <a href="#NT-EntityValue">EntityValue</a> in an entity declaration.</p></div></div><div class="div2"> <h3><a name="intern-replacement" id="intern-replacement"/>4.5 Construction of Entity Replacement Text</h3><p>In discussing the treatment of entities, it is useful to distinguish two forms of the entity's value. [<a name="dt-litentval" id="dt-litentval" title="Literal Entity Value">Definition</a>: For an internal entity, the <b>literal entity value</b> is the quoted string actually present in the entity declaration, corresponding to the non-terminal <a href="#NT-EntityValue">EntityValue</a>.] [<a name="dt-extlitentval" id="dt-extlitentval" title="Literal Entity Value">Definition</a>: For an external entity, the <b>literal entity value</b> is the exact text contained in the entity.] [<a name="dt-repltext" id="dt-repltext" title="Replacement Text">Definition</a>: For an internal entity, the <b>replacement text</b> is the content of the entity, after replacement of character references and parameter-entity references.] [<a name="dt-extrepltext" id="dt-extrepltext" title="Replacement Text">Definition</a>: For an external entity, the <b>replacement text</b> is the content of the entity, after stripping the text declaration (leaving any surrounding whitespace) if there is one but without any replacement of character references or parameter-entity references.] </p><p>The literal entity value as given in an internal entity declaration (<a href="#NT-EntityValue">EntityValue</a>) may contain character, parameter-entity, and general-entity references. Such references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be contained entirely within the literal entity value. The actual replacement text that is <a title="Include" href="#dt-include">included</a> (or <a title="" href="#inliteral">included in literal</a>) as described above <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> contain the <em>replacement text</em> of any parameter entities referred to, and <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> contain the character referred to, in place of any character references in the literal entity value; however, general-entity references <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be left as-is, unexpanded. For example, given the following declarations:</p><div class="exampleInner"><pre><!ENTITY % pub "&#xc9;ditions Gallimard" > <!ENTITY rights "All rights reserved" > <!ENTITY book "La Peste: Albert Camus, &#xA9; 1947 %pub;. &rights;" ></pre></div><p>then the replacement text for the entity " <code>book</code> " is:</p><div class="exampleInner"><pre>La Peste: Albert Camus, © 1947 Éditions Gallimard. &rights;</pre></div><p>The general-entity reference " <code>&rights;</code> " would be expanded should the reference " <code>&book;</code> " appear in the document's content or an attribute value.</p><p>These simple rules may have complex interactions; for a detailed discussion of a difficult example, see <a href="#sec-entexpand"><b>D Expansion of Entity and Character References</b></a>.</p></div><div class="div2"> <h3><a name="sec-predefined-ent" id="sec-predefined-ent"/>4.6 Predefined Entities</h3><p> [<a name="dt-escape" id="dt-escape" title="escape">Definition</a>: Entity and character references may both be used to <b>escape</b> the left angle bracket, ampersand, and other delimiters. A set of general entities (<code>amp</code>, <code>lt</code>, <code>gt</code>, <code>apos</code>, <code>quot</code>) is specified for this purpose. Numeric character references may also be used; they are expanded immediately when recognized and <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be treated as character data, so the numeric character references " <code>&#60;</code> " and " <code>&#38;</code> " may be used to escape <code><</code> and <code>&</code> when they occur in character data.] </p><p>All XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> recognize these entities whether they are declared or not. <a title="For interoperability" href="#dt-interop">For interoperability</a>, valid XML documents <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> declare these entities, like any others, before using them. If the entities <code>lt</code> or <code>amp</code> are declared, they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be declared as internal entities whose replacement text is a character reference to the respective character (less-than sign or ampersand) being escaped; the double escaping is <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em> for these entities so that references to them produce a well-formed result. If the entities <code>gt</code>, <code>apos</code>, or <code>quot</code> are declared, they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> be declared as internal entities whose replacement text is the single character being escaped (or a character reference to that character; the double escaping here is <em class="rfc2119" title="Keyword in RFC 2119 context">OPTIONAL</em> but harmless). For example:</p><div class="exampleInner"><pre><!ENTITY lt "&#38;#60;"> <!ENTITY gt "&#62;"> <!ENTITY amp "&#38;#38;"> <!ENTITY apos "&#39;"> <!ENTITY quot "&#34;"></pre></div></div><div class="div2"> <h3><a name="Notations" id="Notations"/>4.7 Notation Declarations</h3><p> [<a name="dt-notation" id="dt-notation" title="Notation">Definition</a>: <b>Notations</b> identify by name the format of <a title="Unparsed Entity" href="#dt-unparsed">unparsed entities</a>, the format of elements which bear a notation attribute, or the application to which a <a title="Processing instruction" href="#dt-pi">processing instruction</a> is addressed.] </p><p> [<a name="dt-notdecl" id="dt-notdecl" title="Notation Declaration">Definition</a>: <b>Notation declarations</b> provide a name for the notation, for use in entity and attribute-list declarations and in attribute specifications, and an external identifier for the notation which may allow an XML processor or its client application to locate a helper application capable of processing data in the given notation.] </p> <h5><a name="d0e5457" id="d0e5457"/>Notation Declarations</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-NotationDecl" id="NT-NotationDecl"/>[82] </td><td><code>NotationDecl</code></td><td> ::= </td><td><code>'<!NOTATION' <a href="#NT-S">S</a> <a href="#NT-Name">Name</a> <a href="#NT-S">S</a> (<a href="#NT-ExternalID">ExternalID</a> | <a href="#NT-PublicID">PublicID</a>) <a href="#NT-S">S</a>? '>'</code></td><td><a href="#UniqueNotationName">[VC: Unique Notation Name]</a></td></tr></tbody><tbody><tr valign="baseline"><td><a name="NT-PublicID" id="NT-PublicID"/>[83] </td><td><code>PublicID</code></td><td> ::= </td><td><code>'PUBLIC' <a href="#NT-S">S</a> <a href="#NT-PubidLiteral">PubidLiteral</a> </code></td></tr></tbody></table><div class="constraint"><p class="prefix"><a name="UniqueNotationName" id="UniqueNotationName"/><b>Validity constraint: Unique Notation Name</b></p><p>A given <a href="#NT-Name">Name</a> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> be declared in more than one notation declaration.</p></div><p>XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> provide applications with the name and external identifier(s) of any notation declared and referred to in an attribute value, attribute definition, or entity declaration. They <em class="rfc2119" title="Keyword in RFC 2119 context">MAY</em> additionally resolve the external identifier into the <a title="System Identifier" href="#dt-sysid">system identifier</a>, file name, or other information needed to allow the application to call a processor for data in the notation described. (It is not an error, however, for XML documents to declare and refer to notations for which notation-specific applications are not available on the system where the XML processor or application is running.)</p></div><div class="div2"> <h3><a name="sec-doc-entity" id="sec-doc-entity"/>4.8 Document Entity</h3><p> [<a name="dt-docent" id="dt-docent" title="Document Entity">Definition</a>: The <b>document entity</b> serves as the root of the entity tree and a starting-point for an <a title="XML Processor" href="#dt-xml-proc">XML processor</a>.] This specification does not specify how the document entity is to be located by an XML processor; unlike other entities, the document entity has no name and might well appear on a processor input stream without any identification at all.</p></div></div><div class="div1"> <h2><a name="sec-conformance" id="sec-conformance"/>5 Conformance</h2><div class="div2"> <h3><a name="proc-types" id="proc-types"/>5.1 Validating and Non-Validating Processors</h3><p>Conforming <a title="XML Processor" href="#dt-xml-proc">XML processors</a> fall into two classes: validating and non-validating.</p><p>Validating and non-validating processors alike <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> report violations of this specification's well-formedness constraints in the content of the <a title="Document Entity" href="#dt-docent">document entity</a> and any other <a title="Text Entity" href="#dt-parsedent">parsed entities</a> that they read.</p><p> [<a name="dt-validating" id="dt-validating" title="Validating Processor">Definition</a>: <b>Validating processors</b> <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em>, at user option, report violations of the constraints expressed by the declarations in the <a title="Document Type Declaration" href="#dt-doctype">DTD</a>, and failures to fulfill the validity constraints given in this specification.] To accomplish this, validating XML processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> read and process the entire DTD and all external parsed entities referenced in the document.</p><p>Non-validating processors are <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em> to check only the <a title="Document Entity" href="#dt-docent">document entity</a>, including the entire internal DTD subset, for well-formedness. [<a name="dt-use-mdecl" id="dt-use-mdecl" title="Process Declarations">Definition</a>: While they are not required to check the document for validity, they are <em class="rfc2119" title="Keyword in RFC 2119 context">REQUIRED</em> to <b>process</b> all the declarations they read in the internal DTD subset and in any parameter entity that they read, up to the first reference to a parameter entity that they do <em>not</em> read; that is to say, they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> use the information in those declarations to <a href="#AVNormalize"><cite>normalize</cite></a> attribute values, <a href="#included"><cite>include</cite></a> the replacement text of internal entities, and supply <a href="#sec-attr-defaults"><cite>default attribute values</cite></a>.] Except when <code>standalone="yes"</code>, they <em class="rfc2119" title="Keyword in RFC 2119 context">MUST NOT</em> <a title="Process Declarations" href="#dt-use-mdecl">process</a> <a title="entity declaration" href="#dt-entdecl">entity declarations</a> or <a title="Attribute-List Declaration" href="#dt-attdecl">attribute-list declarations</a> encountered after a reference to a parameter entity that is not read, since the entity may have contained overriding declarations; when <code>standalone="yes"</code>, processors <em class="rfc2119" title="Keyword in RFC 2119 context">MUST</em> process these declarations.</p><p>Note that when processing invalid documents with a non-validating processor the application may not be presented with consistent information. For example, several requirements for uniqueness within the document may not be met, including more than one element with the same id, duplicate declarations of elements or notations with the same name, etc. In these cases the behavior of the parser with respect to reporting such information to the application is undefined.</p></div><div class="div2"> <h3><a name="safe-behavior" id="safe-behavior"/>5.2 Using XML Processors</h3><p>The behavior of a validating XML processor is highly predictable; it must read every piece of a document and report all well-formedness and validity violations. Less is required of a non-validating processor; it need not read any part of the document other than the document entity. This has two effects that may be important to users of XML processors:</p><ul><li><p>Certain well-formedness errors, specifically those that require reading external entities, may fail to be detected by a non-validating processor. Examples include the constraints entitled <a href="#wf-entdeclared"><cite>Entity Declared</cite></a>, <a href="#textent"><cite>Parsed Entity</cite></a>, and <a href="#norecursion"><cite>No Recursion</cite></a>, as well as some of the cases described as <a href="#forbidden"><cite>forbidden</cite></a> in <a href="#entproc"><b>4.4 XML Processor Treatment of Entities and References</b></a>.</p></li><li><p>The information passed from the processor to the application may vary, depending on whether the processor reads parameter and external entities. For example, a non-validating processor may fail to <a href="#AVNormalize"><cite>normalize</cite></a> attribute values, <a href="#included"><cite>include</cite></a> the replacement text of internal entities, or supply <a href="#sec-attr-defaults"><cite>default attribute values</cite></a>, where doing so depends on having read declarations in external or parameter entities<span>, or in the internal subset after an unread parameter entity reference</span>.</p></li></ul><p>For maximum reliability in interoperating between different XML processors, applications which use non-validating processors <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD NOT</em> rely on any behaviors not required of such processors. Applications which require DTD facilities not related to validation (such as the declaration of default attributes and internal entities that are or may be specified in external entities) <em class="rfc2119" title="Keyword in RFC 2119 context">SHOULD</em> use validating XML processors.</p></div></div><div class="div1"> <h2><a name="sec-notation" id="sec-notation"/>6 Notation</h2><p>The formal grammar of XML is given in this specification using a simple Extended Backus-Naur Form (EBNF) notation. Each rule in the grammar defines one symbol, in the form</p><div class="exampleInner"><pre>symbol ::= expression</pre></div><p>Symbols are written with an initial capital letter if they are the start symbol of a regular language, otherwise with an initial lowercase letter. Literal strings are quoted.</p><p>Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters: </p><dl><dt class="label"> <code>#xN</code> </dt><dd><p>where <code>N</code> is a hexadecimal integer, the expression matches the character whose number (code point) in ISO/IEC 10646 is <code>N</code>. The number of leading zeros in the <code>#xN</code> form is insignificant.</p></dd><dt class="label"> <code>[a-zA-Z]</code>, <code>[#xN-#xN]</code> </dt><dd><p>matches any <a href="#NT-Char">Char</a> with a value in the range(s) indicated (inclusive).</p></dd><dt class="label"> <code>[abc]</code>, <code>[#xN#xN#xN]</code> </dt><dd><p>matches any <a href="#NT-Char">Char</a> with a value among the characters enumerated. Enumerations and ranges can be mixed in one set of brackets.</p></dd><dt class="label"> <code>[^a-z]</code>, <code>[^#xN-#xN]</code> </dt><dd><p>matches any <a href="#NT-Char">Char</a> with a value <em>outside</em> the range indicated.</p></dd><dt class="label"> <code>[^abc]</code>, <code>[^#xN#xN#xN]</code> </dt><dd><p>matches any <a href="#NT-Char">Char</a> with a value not among the characters given. Enumerations and ranges of forbidden values can be mixed in one set of brackets.</p></dd><dt class="label"> <code>"string"</code> </dt><dd><p>matches a literal string <a title="match" href="#dt-match">matching</a> that given inside the double quotes.</p></dd><dt class="label"> <code>'string'</code> </dt><dd><p>matches a literal string <a title="match" href="#dt-match">matching</a> that given inside the single quotes.</p></dd></dl><p> These symbols may be combined to match more complex patterns as follows, where <code>A</code> and <code>B</code> represent simple expressions: </p><dl><dt class="label">(<code>expression</code>)</dt><dd><p> <code>expression</code> is treated as a unit and may be combined as described in this list.</p></dd><dt class="label"> <code>A?</code> </dt><dd><p>matches <code>A</code> or nothing; optional <code>A</code>.</p></dd><dt class="label"> <code>A B</code> </dt><dd><p>matches <code>A</code> followed by <code>B</code>. This operator has higher precedence than alternation; thus <code>A B | C D</code> is identical to <code>(A B) | (C D)</code>.</p></dd><dt class="label"> <code>A | B</code> </dt><dd><p>matches <code>A</code> or <code>B</code>.</p></dd><dt class="label"> <code>A - B</code> </dt><dd><p>matches any string that matches <code>A</code> but does not match <code>B</code>.</p></dd><dt class="label"> <code>A+</code> </dt><dd><p>matches one or more occurrences of <code>A</code>. Concatenation has higher precedence than alternation; thus <code>A+ | B+</code> is identical to <code>(A+) | (B+)</code>.</p></dd><dt class="label"> <code>A*</code> </dt><dd><p>matches zero or more occurrences of <code>A</code>. Concatenation has higher precedence than alternation; thus <code>A* | B*</code> is identical to <code>(A*) | (B*)</code>.</p></dd></dl><p> Other notations used in the productions are: </p><dl><dt class="label"> <code>/* ... */</code> </dt><dd><p>comment.</p></dd><dt class="label"> <code>[ wfc: ... ]</code> </dt><dd><p>well-formedness constraint; this identifies by name a constraint on <a title="Well-Formed" href="#dt-wellformed">well-formed</a> documents associated with a production.</p></dd><dt class="label"> <code>[ vc: ... ]</code> </dt><dd><p>validity constraint; this identifies by name a constraint on <a title="Validity" href="#dt-valid">valid</a> documents associated with a production.</p></dd></dl><p> </p></div></div><div class="back"><div class="div1"> <h2><a name="sec-bibliography" id="sec-bibliography"/>A References</h2><div class="div2"> <h3><a name="sec-existing-stds" id="sec-existing-stds"/>A.1 Normative References</h3><dl><dt class="label"><a name="IANA" id="IANA"/>IANA-CHARSETS</dt><dd>(Internet Assigned Numbers Authority) <a href="http://www.iana.org/assignments/character-sets"><cite>Official Names for Character Sets</cite></a>, ed. Keld Simonsen et al. (See http://www.iana.org/assignments/character-sets.)</dd><dt class="label"><a name="rfc2119" id="rfc2119"/>IETF RFC 2119</dt><dd>IETF (Internet Engineering Task Force). <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>RFC 2119: Key words for use in RFCs to Indicate Requirement Levels</cite></a>. Scott Bradner, 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)</dd><dt class="label"><a name="RFC1766"/>IETF BCP 47</dt><dd>IETF (Internet Engineering Task Force). <cite>BCP 47, consisting of <a href="http://tools.ietf.org/html/rfc4646">RFC 4646: Tags for Identifying Languages</a>, and <a href="http://tools.ietf.org/html/rfc4647">RFC 4647: Matching of Language Tags</a></cite>, A. Phillips, M. Davis. 2006.</dd><dt class="label"><a name="rfc3986" id="rfc3986"/>IETF RFC 3986</dt><dd>IETF (Internet Engineering Task Force). <a href="http://www.ietf.org/rfc/rfc3986.txt"><cite>RFC 3986: Uniform Resource Identifier (URI): Generic Syntax</cite></a>. T. Berners-Lee, R. Fielding, L. Masinter. 2005. (See http://www.ietf.org/rfc/rfc3986.txt.)</dd><dt class="label"><a name="ISO10646" id="ISO10646"/>ISO/IEC 10646</dt><dd>ISO (International Organization for Standardization). <cite>ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane</cite> and <cite>ISO/IEC 10646-2:2001. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 2: Supplementary Planes</cite>, as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. [Geneva]: International Organization for Standardization. (See <a href="http://www.iso.org/iso/home.htm">http://www.iso.org/iso/home.htm</a> for the latest version.)</dd><dt class="label"><a name="ISO10646-2000" id="ISO10646-2000"/>ISO/IEC 10646:2000</dt><dd>ISO (International Organization for Standardization). <cite>ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane.</cite> [Geneva]: International Organization for Standardization, 2000.</dd><dt class="label"><a name="Unicode" id="Unicode"/>Unicode</dt><dd>The Unicode Consortium. <em>The Unicode Standard, Version <span>5.0.0,</span></em> <span>defined by: The Unicode Standard, Version 5.0 (Boston, MA, Addison-Wesley, 2007. ISBN 0-321-48091-0)</span>.</dd><dt class="label"><a name="unipart15" id="unipart15"/>UnicodeNormal</dt><dd>The Unicode Consortium. <a href="http://unicode.org/reports/tr15/"><cite>Unicode normalization forms</cite></a>. Mark Davis and Martin Durst. 2008. (See http://unicode.org/reports/tr15/.)</dd></dl></div><div class="div2"> <h3><a name="null" id="null"/>A.2 Other References</h3><dl><dt class="label"><a name="Aho" id="Aho"/>Aho/Ullman</dt><dd>Aho, Alfred V., Ravi Sethi, and Jeffrey D. Ullman. <cite>Compilers: Principles, Techniques, and Tools</cite>. Reading: Addison-Wesley, 1986, rpt. corr. 1988.</dd><dt class="label"><a name="ABK" id="ABK"/>Brüggemann-Klein</dt><dd>Brüggemann-Klein, Anne. <a href="ftp://ftp.informatik.uni-freiburg.de/documents/papers/brueggem/habil.ps"><cite>Formal Models in Document Processing</cite></a>. Habilitationsschrift. Faculty of Mathematics at the University of Freiburg, 1993. (See ftp://ftp.informatik.uni-freiburg.de/documents/papers/brueggem/habil.ps.)</dd><dt class="label"><a name="ABKDW" id="ABKDW"/>Brüggemann-Klein and Wood</dt><dd>Brüggemann-Klein, Anne, and Derick Wood. <cite>Deterministic Regular Languages</cite>. Universität Freiburg, Institut für Informatik, Bericht 38, Oktober 1991. Extended abstract in A. Finkel, M. Jantzen, Hrsg., STACS 1992, S. 173-184. Springer-Verlag, Berlin 1992. Lecture Notes in Computer Science 577. Full version titled <cite>One-Unambiguous Regular Languages</cite> in Information and Computation 140 (2): 229-253, February 1998.</dd><dt class="label"><a name="Clark" id="Clark"/>Clark</dt><dd>James Clark. <a href="https://www.w3.org/TR/NOTE-sgml-xml-971215"><cite>Comparison of SGML and XML</cite></a>. (See http://www.w3.org/TR/NOTE-sgml-xml-971215.)</dd><dt class="label"><a name="IANA-LANGCODES"/>IANA-LANGCODES</dt><dd>(Internet Assigned Numbers Authority) <a href="http://www.iana.org/assignments/language-subtag-registry"><cite>Registry of Language Tags</cite></a> (See http://www.iana.org/assignments/language-subtag-registry.)</dd><dt class="label"><a name="RFC2141" id="RFC2141"/>IETF RFC 2141</dt><dd>IETF (Internet Engineering Task Force). <a href="http://www.ietf.org/rfc/rfc2141.txt"><cite>RFC 2141: URN Syntax</cite></a>, ed. R. Moats. 1997. (See http://www.ietf.org/rfc/rfc2141.txt.)</dd><dt class="label"><a name="rfc2376" id="rfc2376"/>IETF RFC 3023</dt><dd>IETF (Internet Engineering Task Force). <a href="http://www.ietf.org/rfc/rfc3023.txt"><cite>RFC 3023: XML Media Types</cite></a>. eds. M. Murata, S. St.Laurent, D. Kohn. 2001. (See http://www.ietf.org/rfc/rfc3023.txt.)</dd><dt class="label"><a name="rfc2781" id="rfc2781"/>IETF RFC 2781</dt><dd>IETF (Internet Engineering Task Force). <a href="http://www.ietf.org/rfc/rfc2781.txt"><cite>RFC 2781: UTF-16, an encoding of ISO 10646</cite></a>, ed. P. Hoffman, F. Yergeau. 2000. (See http://www.ietf.org/rfc/rfc2781.txt.)</dd><dt class="label"><a name="ISO639" id="ISO639"/>ISO 639</dt><dd>(International Organization for Standardization). <cite>ISO 639:1988 (E). Code for the representation of names of languages.</cite> [Geneva]: International Organization for Standardization, 1988.</dd><dt class="label"><a name="ISO3166" id="ISO3166"/>ISO 3166</dt><dd>(International Organization for Standardization). <cite>ISO 3166-1:1997 (E). Codes for the representation of names of countries and their subdivisions — Part 1: Country codes</cite> [Geneva]: International Organization for Standardization, 1997.</dd><dt class="label"><a name="ISO8879" id="ISO8879"/>ISO 8879</dt><dd>ISO (International Organization for Standardization). <cite>ISO 8879:1986(E). Information processing — Text and Office Systems — Standard Generalized Markup Language (SGML).</cite> First edition — 1986-10-15. [Geneva]: International Organization for Standardization, 1986. </dd><dt class="label"><a name="ISO10744" id="ISO10744"/>ISO/IEC 10744</dt><dd>ISO (International Organization for Standardization). <cite>ISO/IEC 10744-1992 (E). Information technology — Hypermedia/Time-based Structuring Language (HyTime). </cite> [Geneva]: International Organization for Standardization, 1992. <em>Extended Facilities Annexe.</em> [Geneva]: International Organization for Standardization, 1996. </dd><dt class="label"><a name="websgml" id="websgml"/>WEBSGML</dt><dd>ISO (International Organization for Standardization). <a href="http://www.sgmlsource.com/8879/n0029.htm"><cite>ISO 8879:1986 TC2. Information technology — Document Description and Processing Languages</cite></a>. [Geneva]: International Organization for Standardization, 1998. (See http://www.sgmlsource.com/8879/n0029.htm.)</dd><dt class="label"><a name="xml-names" id="xml-names"/>XML Names</dt><dd>Tim Bray, Dave Hollander, and Andrew Layman, editors. <a href="https://www.w3.org/TR/xml-names/"><cite>Namespaces in XML</cite></a>. Textuality, Hewlett-Packard, and Microsoft. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/xml-names/.)</dd></dl></div></div><div class="div1"> <h2><a name="CharClasses" id="CharClasses"/>B Character Classes</h2><p>Because of changes to productions <a href="#NT-NameStartChar">[4]</a> and <a href="#NT-Name">[5]</a>, the productions in this Appendix are now orphaned and not used anymore in determining name characters. This Appendix may be removed in a future edition of this specification; other specifications that wish to refer to the productions herein should do so by means of a reference to the relevant production(s) in the <a href="https://www.w3.org/TR/2006/REC-xml-20060816/">Fourth Edition</a> of this specification. </p><p>Following the characteristics defined in the Unicode standard, characters are classed as base characters (among others, these contain the alphabetic characters of the Latin alphabet), ideographic characters, and combining characters (among others, this class contains most diacritics). Digits and extenders are also distinguished.</p> <h5><a name="CHARACTERS" id="CHARACTERS"/>Characters</h5><table class="scrap" summary="Scrap"><tbody><tr valign="baseline"><td><a name="NT-Letter" id="NT-Letter"/>[84] </td><td><code>Letter</code></td><td> ::= </td><td><code> <a href="#NT-BaseChar">BaseChar</a> | <a href="#NT-Ideographic">Ideographic</a> </code></td></tr><tr valign="baseline"><td><a name="NT-BaseChar" id="NT-BaseChar"/>[85] </td><td><code>BaseChar</code></td><td> ::= </td><td><code>[#x0041-#x005A] | [#x0061-#x007A] | [#x00C0-#x00D6] | [#x00D8-#x00F6] | [#x00F8-#x00FF] | [#x0100-#x0131] | [#x0134-#x013E] | [#x0141-#x0148] | [#x014A-#x017E] | [#x0180-#x01C3] | [#x01CD-#x01F0] | [#x01F4-#x01F5] | [#x01FA-#x0217] | [#x0250-#x02A8] | [#x02BB-#x02C1] | #x0386 | [#x0388-#x038A] | #x038C | [#x038E-#x03A1] | [#x03A3-#x03CE] | [#x03D0-#x03D6] | #x03DA | #x03DC | #x03DE | #x03E0 | [#x03E2-#x03F3] | [#x0401-#x040C] | [#x040E-#x044F] | [#x0451-#x045C] | [#x045E-#x0481] | [#x0490-#x04C4] | [#x04C7-#x04C8] | [#x04CB-#x04CC] | [#x04D0-#x04EB] | [#x04EE-#x04F5] | [#x04F8-#x04F9] | [#x0531-#x0556] | #x0559 | [#x0561-#x0586] | [#x05D0-#x05EA] | [#x05F0-#x05F2] | [#x0621-#x063A] | [#x0641-#x064A] | [#x0671-#x06B7] | [#x06BA-#x06BE] | [#x06C0-#x06CE] | [#x06D0-#x06D3] | #x06D5 | [#x06E5-#x06E6] | [#x0905-#x0939] | #x093D | [#x0958-#x0961] | [#x0985-#x098C] | [#x098F-#x0990] | [#x0993-#x09A8] | [#x09AA-#x09B0] | #x09B2 | [#x09B6-#x09B9] | [#x09DC-#x09DD] | [#x09DF-#x09E1] | [#x09F0-#x09F1] | [#x0A05-#x0A0A] | [#x0A0F-#x0A10] | [#x0A13-#x0A28] | [#x0A2A-#x0A30] | [#x0A32-#x0A33] | [#x0A35-#x0A36] | [#x0A38-#x0A39] | [#x0A59-#x0A5C] | #x0A5E | [#x0A72-#x0A74] | [#x0A85-#x0A8B] | #x0A8D | [#x0A8F-#x0A91] | [#x0A93-#x0AA8] | [#x0AAA-#x0AB0] | [#x0AB2-#x0AB3] | [#x0AB5-#x0AB9] | #x0ABD | #x0AE0 | [#x0B05-#x0B0C] | [#x0B0F-#x0B10] | [#x0B13-#x0B28] | [#x0B2A-#x0B30] | [#x0B32-#x0B33] | [#x0B36-#x0B39] | #x0B3D | [#x0B5C-#x0B5D] | [#x0B5F-#x0B61] | [#x0B85-#x0B8A] | [#x0B8E-#x0B90] | [#x0B92-#x0B95] | [#x0B99-#x0B9A] | #x0B9C | [#x0B9E-#x0B9F] | [#x0BA3-#x0BA4] | [#x0BA8-#x0BAA] | [#x0BAE-#x0BB5] | [#x0BB7-#x0BB9] | [#x0C05-#x0C0C] | [#x0C0E-#x0C10] | [#x0C12-#x0C28] | [#x0C2A-#x0C33] | [#x0C35-#x0C39] | [#x0C60-#x0C61] | [#x0C85-#x0C8C] | [#x0C8E-#x0C90] | [#x0C92-#x0CA8] | [#x0CAA-#x0CB3] | [#x0CB5-#x0CB9] | #x0CDE | [#x0CE0-#x0CE1] | [#x0D05-#x0D0C] | [#x0D0E-#x0D10] | [#x0D12-#x0D28] | [#x0D2A-#x0D39] | [#x0D60-#x0D61] | [#x0E01-#x0E2E] | #x0E30 | [#x0E32-#x0E33] | [#x0E40-#x0E45] | [#x0E81-#x0E82] | #x0E84 | [#x0E87-#x0E88] | #x0E8A | #x0E8D | [#x0E94-#x0E97] | [#x0E99-#x0E9F] | [#x0EA1-#x0EA3] | #x0EA5 | #x0EA7 | [#x0EAA-#x0EAB] | [#x0EAD-#x0EAE] | #x0EB0 | [#x0EB2-#x0EB3] | #x0EBD | [#x0EC0-#x0EC4] | [#x0F40-#x0F47] | [#x0F49-#x0F69] | [#x10A0-#x10C5] | [#x10D0-#x10F6] | #x1100 | [#x1102-#x1103] | [#x1105-#x1107] | #x1109 | [#x110B-#x110C] | [#x110E-#x1112] | #x113C | #x113E | #x1140 | #x114C | #x114E | #x1150 | [#x1154-#x1155] | #x1159 | [#x115F-#x1161] | #x1163 | #x1165 | #x1167 | #x1169 | [#x116D-#x116E] | [#x1172-#x1173] | #x1175 | #x119E | #x11A8 | #x11AB | [#x11AE-#x11AF] | [#x11B7-#x11B8] | #x11BA | [#x11BC-#x11C2] | #x11EB | #x11F0 | #x11F9 | [#x1E00-#x1E9B] | [#x1EA0-#x1EF9] | [#x1F00-#x1F15] | [#x1F18-#x1F1D] | [#x1F20-#x1F45] | [#x1F48-#x1F4D] | [#x1F50-#x1F57] | #x1F59 | #x1F5B | #x1F5D | [#x1F5F-#x1F7D] | [#x1F80-#x1FB4] | [#x1FB6-#x1FBC] | #x1FBE | [#x1FC2-#x1FC4] | [#x1FC6-#x1FCC] | [#x1FD0-#x1FD3] | [#x1FD6-#x1FDB] | [#x1FE0-#x1FEC] | [#x1FF2-#x1FF4] | [#x1FF6-#x1FFC] | #x2126 | [#x212A-#x212B] | #x212E | [#x2180-#x2182] | [#x3041-#x3094] | [#x30A1-#x30FA] | [#x3105-#x312C] | [#xAC00-#xD7A3] </code></td></tr><tr valign="baseline"><td><a name="NT-Ideographic" id="NT-Ideographic"/>[86] </td><td><code>Ideographic</code></td><td> ::= </td><td><code>[#x4E00-#x9FA5] | #x3007 | [#x3021-#x3029] </code></td></tr><tr valign="baseline"><td><a name="NT-CombiningChar" id="NT-CombiningChar"/>[87] </td><td><code>CombiningChar</code></td><td> ::= </td><td><code>[#x0300-#x0345] | [#x0360-#x0361] | [#x0483-#x0486] | [#x0591-#x05A1] | [#x05A3-#x05B9] | [#x05BB-#x05BD] | #x05BF | [#x05C1-#x05C2] | #x05C4 | [#x064B-#x0652] | #x0670 | [#x06D6-#x06DC] | [#x06DD-#x06DF] | [#x06E0-#x06E4] | [#x06E7-#x06E8] | [#x06EA-#x06ED] | [#x0901-#x0903] | #x093C | [#x093E-#x094C] | #x094D | [#x0951-#x0954] | [#x0962-#x0963] | [#x0981-#x0983] | #x09BC | #x09BE | #x09BF | [#x09C0-#x09C4] | [#x09C7-#x09C8] | [#x09CB-#x09CD] | #x09D7 | [#x09E2-#x09E3] | #x0A02 | #x0A3C | #x0A3E | #x0A3F | [#x0A40-#x0A42] | [#x0A47-#x0A48] | [#x0A4B-#x0A4D] | [#x0A70-#x0A71] | [#x0A81-#x0A83] | #x0ABC | [#x0ABE-#x0AC5] | [#x0AC7-#x0AC9] | [#x0ACB-#x0ACD] | [#x0B01-#x0B03] | #x0B3C | [#x0B3E-#x0B43] | [#x0B47-#x0B48] | [#x0B4B-#x0B4D] | [#x0B56-#x0B57] | [#x0B82-#x0B83] | [#x0BBE-#x0BC2] | [#x0BC6-#x0BC8] | [#x0BCA-#x0BCD] | #x0BD7 | [#x0C01-#x0C03] | [#x0C3E-#x0C44] | [#x0C46-#x0C48] | [#x0C4A-#x0C4D] | [#x0C55-#x0C56] | [#x0C82-#x0C83] | [#x0CBE-#x0CC4] | [#x0CC6-#x0CC8] | [#x0CCA-#x0CCD] | [#x0CD5-#x0CD6] | [#x0D02-#x0D03] | [#x0D3E-#x0D43] | [#x0D46-#x0D48] | [#x0D4A-#x0D4D] | #x0D57 | #x0E31 | [#x0E34-#x0E3A] | [#x0E47-#x0E4E] | #x0EB1 | [#x0EB4-#x0EB9] | [#x0EBB-#x0EBC] | [#x0EC8-#x0ECD] | [#x0F18-#x0F19] | #x0F35 | #x0F37 | #x0F39 | #x0F3E | #x0F3F | [#x0F71-#x0F84] | [#x0F86-#x0F8B] | [#x0F90-#x0F95] | #x0F97 | [#x0F99-#x0FAD] | [#x0FB1-#x0FB7] | #x0FB9 | [#x20D0-#x20DC] | #x20E1 | [#x302A-#x302F] | #x3099 | #x309A </code></td></tr><tr valign="baseline"><td><a name="NT-Digit" id="NT-Digit"/>[88] </td><td><code>Digit</code></td><td> ::= </td><td><code>[#x0030-#x0039] | [#x0660-#x0669] | [#x06F0-#x06F9] | [#x0966-#x096F] | [#x09E6-#x09EF] | [#x0A66-#x0A6F] | [#x0AE6-#x0AEF] | [#x0B66-#x0B6F] | [#x0BE7-#x0BEF] | [#x0C66-#x0C6F] | [#x0CE6-#x0CEF] | [#x0D66-#x0D6F] | [#x0E50-#x0E59] | [#x0ED0-#x0ED9] | [#x0F20-#x0F29] </code></td></tr><tr valign="baseline"><td><a name="NT-Extender" id="NT-Extender"/>[89] </td><td><code>Extender</code></td><td> ::= </td><td><code>#x00B7 | #x02D0 | #x02D1 | #x0387 | #x0640 | #x0E46 | #x0EC6 | #x3005 | [#x3031-#x3035] | [#x309D-#x309E] | [#x30FC-#x30FE] </code></td></tr></tbody></table><p>The character classes defined here can be derived from the Unicode 2.0 character database as follows:</p><ul><li><p>Name start characters must have one of the categories Ll, Lu, Lo, Lt, Nl.</p></li><li><p>Name characters other than Name-start characters must have one of the categories Mc, Me, Mn, Lm, or Nd.</p></li><li><p>Characters in the compatibility area (i.e. with character code greater than #xF900 and less than #xFFFE) are not allowed in XML names.</p></li><li><p>Characters which have a font or compatibility decomposition (i.e. those with a "compatibility formatting tag" in field 5 of the database -- marked by field 5 beginning with a "<") are not allowed.</p></li><li><p>The following characters are treated as name-start characters rather than name characters, because the property file classifies them as Alphabetic: [#x02BB-#x02C1], #x0559, #x06E5, #x06E6.</p></li><li><p>Characters #x20DD-#x20E0 are excluded (in accordance with Unicode 2.0, section 5.14).</p></li><li><p>Character #x00B7 is classified as an extender, because the property list so identifies it.</p></li><li><p>Character #x0387 is added as a name character, because #x00B7 is its canonical equivalent.</p></li><li><p>Characters ':' and '_' are allowed as name-start characters.</p></li><li><p>Characters '-' and '.' are allowed as name characters.</p></li></ul></div><div class="div1"> <h2><a name="sec-xml-and-sgml" id="sec-xml-and-sgml"/>C XML and SGML (Non-Normative)</h2><p>XML is designed to be a subset of SGML, in that every XML document should also be a conforming SGML document. For a detailed comparison of the additional restrictions that XML places on documents beyond those of SGML, see <a href="#Clark">[Clark]</a>.</p></div><div class="div1"> <h2><a name="sec-entexpand" id="sec-entexpand"/>D Expansion of Entity and Character References (Non-Normative)</h2><p>This appendix contains some examples illustrating the sequence of entity- and character-reference recognition and expansion, as specified in <a href="#entproc"><b>4.4 XML Processor Treatment of Entities and References</b></a>.</p><p>If the DTD contains the declaration</p><div class="exampleInner"><pre><!ENTITY example "<p>An ampersand (&#38;#38;) may be escaped numerically (&#38;#38;#38;) or with a general entity (&amp;amp;).</p>" ></pre></div><p>then the XML processor will recognize the character references when it parses the entity declaration, and resolve them before storing the following string as the value of the entity " <code>example</code> ":</p><div class="exampleInner"><pre><p>An ampersand (&#38;) may be escaped numerically (&#38;#38;) or with a general entity (&amp;amp;).</p></pre></div><p>A reference in the document to " <code>&example;</code> " will cause the text to be reparsed, at which time the start- and end-tags of the <code>p</code> element will be recognized and the three references will be recognized and expanded, resulting in a <code>p</code> element with the following content (all data, no delimiters or markup):</p><div class="exampleInner"><pre>An ampersand (&) may be escaped numerically (&#38;) or with a general entity (&amp;).</pre></div><p>A more complex example will illustrate the rules and their effects fully. In the following example, the line numbers are solely for reference.</p><div class="exampleInner"><pre>1 <?xml version='1.0'?> 2 <!DOCTYPE test [ 3 <!ELEMENT test (#PCDATA) > 4 <!ENTITY % xx '&#37;zz;'> 5 <!ENTITY % zz '&#60;!ENTITY tricky "error-prone" >' > 6 %xx; 7 ]> 8 <test>This sample shows a &tricky; method.</test></pre></div><p>This produces the following:</p><ul><li><p>in line 4, the reference to character 37 is expanded immediately, and the parameter entity " <code>xx</code> " is stored in the symbol table with the value " <code>%zz;</code> ". Since the replacement text is not rescanned, the reference to parameter entity " <code>zz</code> " is not recognized. (And it would be an error if it were, since " <code>zz</code> " is not yet declared.)</p></li><li><p>in line 5, the character reference " <code>&#60;</code> " is expanded immediately and the parameter entity " <code>zz</code> " is stored with the replacement text " <code><!ENTITY tricky "error-prone" ></code> ", which is a well-formed entity declaration.</p></li><li><p>in line 6, the reference to " <code>xx</code> " is recognized, and the replacement text of " <code>xx</code> " (namely " <code>%zz;</code> ") is parsed. The reference to " <code>zz</code> " is recognized in its turn, and its replacement text (" <code><!ENTITY tricky "error-prone" ></code> ") is parsed. The general entity " <code>tricky</code> " has now been declared, with the replacement text " <code>error-prone</code> ".</p></li><li><p>in line 8, the reference to the general entity " <code>tricky</code> " is recognized, and it is expanded, so the full content of the <code>test</code> element is the self-describing (and ungrammatical) string <em>This sample shows a error-prone method.</em> </p></li></ul><p>In the following example</p><div class="exampleInner"><pre><!DOCTYPE foo [ <!ENTITY x "&lt;"> ]> <foo attr="&x;"/></pre></div><p>the replacement text of x is the four characters "&lt;" because references to general entities in entity values are <a href="#bypass"><cite>bypassed</cite></a>. The replacement text of lt is a character reference to the less-than character, for example the five characters "&#60;" (see <a href="#sec-predefined-ent"><b>4.6 Predefined Entities</b></a>). Since neither of these contains a less-than character the result is well-formed.</p><p>If the definition of x had been</p><div class="exampleInner"><pre><!ENTITY x "&#60;"></pre></div><p>then the document would not have been well-formed, because the replacement text of x would be the single character "<" which is not permitted in attribute values (see <a href="#CleanAttrVals"><cite>WFC: No < in Attribute Values</cite></a>).</p></div><div class="div1"> <h2><a name="determinism" id="determinism"/>E Deterministic Content Models (Non-Normative)</h2><p>As noted in <a href="#sec-element-content"><b>3.2.1 Element Content</b></a>, it is required that content models in element type declarations be deterministic. This requirement is <a title="For Compatibility" href="#dt-compat">for compatibility</a> with SGML (which calls deterministic content models "unambiguous"); XML processors built using SGML systems may flag non-deterministic content models as errors.</p><p>For example, the content model <code>((b, c) | (b, d))</code> is non-deterministic, because given an initial <code>b</code> the XML processor cannot know which <code>b</code> in the model is being matched without looking ahead to see which element follows the <code>b</code>. In this case, the two references to <code>b</code> can be collapsed into a single reference, making the model read <code>(b, (c | d))</code>. An initial <code>b</code> now clearly matches only a single name in the content model. The processor doesn't need to look ahead to see what follows; either <code>c</code> or <code>d</code> would be accepted.</p><p>More formally: a finite state automaton may be constructed from the content model using the standard algorithms, e.g. algorithm 3.5 in section 3.9 of Aho, Sethi, and Ullman <a href="#Aho">[Aho/Ullman]</a>. In many such algorithms, a follow set is constructed for each position in the regular expression (i.e., each leaf node in the syntax tree for the regular expression); if any position has a follow set in which more than one following position is labeled with the same element type name, then the content model is in error and may be reported as an error.</p><p>Algorithms exist which allow many but not all non-deterministic content models to be reduced automatically to equivalent deterministic models; see Brüggemann-Klein 1991 <a href="#ABK">[Brüggemann-Klein]</a>.</p></div><div class="div1"> <h2><a name="sec-guessing" id="sec-guessing"/>F Autodetection of Character Encodings (Non-Normative)</h2><p>The XML encoding declaration functions as an internal label on each entity, indicating which character encoding is in use. Before an XML processor can read the internal label, however, it apparently has to know what character encoding is in use—which is what the internal label is trying to indicate. In the general case, this is a hopeless situation. It is not entirely hopeless in XML, however, because XML limits the general case in two ways: each implementation is assumed to support only a finite set of character encodings, and the XML encoding declaration is restricted in position and content in order to make it feasible to autodetect the character encoding in use in each entity in normal cases. Also, in many cases other sources of information are available in addition to the XML data stream itself. Two cases may be distinguished, depending on whether the XML entity is presented to the processor without, or with, any accompanying (external) information. We <span>will</span> consider <span>these cases in turn.</span></p><div class="div2"> <h3><a name="sec-guessing-no-ext-info" id="sec-guessing-no-ext-info"/>F.1 Detection Without External Encoding Information</h3><p>Because each XML entity not accompanied by external encoding information and not in UTF-8 or UTF-16 encoding must begin with an XML encoding declaration, in which the first characters must be '<code><?xml</code>', any conforming processor can detect, after two to four octets of input, which of the following cases apply. In reading this list, it may help to know that in UCS-4, '<' is " <code>#x0000003C</code> " and '?' is " <code>#x0000003F</code> ", and the Byte Order Mark required of UTF-16 data streams is " <code>#xFEFF</code> ". The notation <var>##</var> is used to denote any byte value except that two consecutive <var>##</var>s cannot be both 00.</p><p>With a Byte Order Mark:</p><table border="1" frame="border"><tbody><tr><td> <code>00 00 FE FF</code> </td><td>UCS-4, big-endian machine (1234 order)</td></tr><tr><td> <code>FF FE 00 00</code> </td><td>UCS-4, little-endian machine (4321 order)</td></tr><tr><td> <code>00 00 FF FE</code> </td><td>UCS-4, unusual octet order (2143)</td></tr><tr><td> <code>FE FF 00 00</code> </td><td>UCS-4, unusual octet order (3412)</td></tr><tr><td> <code>FE FF ## ##</code> </td><td>UTF-16, big-endian</td></tr><tr><td> <code>FF FE ## ##</code> </td><td>UTF-16, little-endian</td></tr><tr><td> <code>EF BB BF</code> </td><td>UTF-8</td></tr></tbody></table><p>Without a Byte Order Mark:</p><table border="1" frame="border"><tbody><tr><td> <code>00 00 00 3C</code> </td><td rowspan="4">UCS-4 or other encoding with a 32-bit code unit and ASCII characters encoded as ASCII values, in respectively big-endian (1234), little-endian (4321) and two unusual byte orders (2143 and 3412). The encoding declaration must be read to determine which of UCS-4 or other supported 32-bit encodings applies.</td></tr><tr><td> <code>3C 00 00 00</code> </td></tr><tr><td> <code>00 00 3C 00</code> </td></tr><tr><td> <code>00 3C 00 00</code> </td></tr><tr><td> <code>00 3C 00 3F</code> </td><td>UTF-16BE or big-endian ISO-10646-UCS-2 or other encoding with a 16-bit code unit in big-endian order and ASCII characters encoded as ASCII values (the encoding declaration must be read to determine which)</td></tr><tr><td> <code>3C 00 3F 00</code> </td><td>UTF-16LE or little-endian ISO-10646-UCS-2 or other encoding with a 16-bit code unit in little-endian order and ASCII characters encoded as ASCII values (the encoding declaration must be read to determine which)</td></tr><tr><td> <code>3C 3F 78 6D</code> </td><td>UTF-8, ISO 646, ASCII, some part of ISO 8859, Shift-JIS, EUC, or any other 7-bit, 8-bit, or mixed-width encoding which ensures that the characters of ASCII have their normal positions, width, and values; the actual encoding declaration must be read to detect which of these applies, but since all of these encodings use the same bit patterns for the relevant ASCII characters, the encoding declaration itself may be read reliably</td></tr><tr><td> <code>4C 6F A7 94</code> </td><td>EBCDIC (in some flavor; the full encoding declaration must be read to tell which code page is in use)</td></tr><tr><td>Other</td><td>UTF-8 without an encoding declaration, or else the data stream is mislabeled (lacking a required encoding declaration), corrupt, fragmentary, or enclosed in a wrapper of some kind</td></tr></tbody></table><div class="note"><p class="prefix"><b>Note:</b></p><p>In cases above which do not require reading the encoding declaration to determine the encoding, section 4.3.3 still requires that the encoding declaration, if present, be read and that the encoding name be checked to match the actual encoding of the entity. Also, it is possible that new character encodings will be invented that will make it necessary to use the encoding declaration to determine the encoding, in cases where this is not required at present.</p></div><p>This level of autodetection is enough to read the XML encoding declaration and parse the character-encoding identifier, which is still necessary to distinguish the individual members of each family of encodings (e.g. to tell UTF-8 from 8859, and the parts of 8859 from each other, or to distinguish the specific EBCDIC code page in use, and so on).</p><p>Because the contents of the encoding declaration are restricted to characters from the ASCII repertoire (however encoded), a processor can reliably read the entire encoding declaration as soon as it has detected which family of encodings is in use. Since in practice, all widely used character encodings fall into one of the categories above, the XML encoding declaration allows reasonably reliable in-band labeling of character encodings, even when external sources of information at the operating-system or transport-protocol level are unreliable. Character encodings such as UTF-7 that make overloaded usage of ASCII-valued bytes may fail to be reliably detected.</p><p>Once the processor has detected the character encoding in use, it can act appropriately, whether by invoking a separate input routine for each case, or by calling the proper conversion function on each character of input.</p><p>Like any self-labeling system, the XML encoding declaration will not work if any software changes the entity's character set or encoding without updating the encoding declaration. Implementors of character-encoding routines should be careful to ensure the accuracy of the internal and external information used to label the entity.</p></div><div class="div2"> <h3><a name="sec-guessing-with-ext-info" id="sec-guessing-with-ext-info"/>F.2 Priorities in the Presence of External Encoding Information</h3><p>The second possible case occurs when the XML entity is accompanied by encoding information, as in some file systems and some network protocols. When multiple sources of information are available, their relative priority and the preferred method of handling conflict should be specified as part of the higher-level protocol used to deliver XML. In particular, please refer to <a href="#rfc2376">[IETF RFC 3023]</a> or its successor, which defines the <code>text/xml</code> and <code>application/xml</code> MIME types and provides some useful guidance. In the interests of interoperability, however, the following rule is recommended.</p><ul><li><p>If an XML entity is in a file, the Byte-Order Mark and encoding declaration are used (if present) to determine the character encoding.</p></li></ul></div></div><div class="div1"> <h2><a name="sec-xml-wg" id="sec-xml-wg"/>G W3C XML Working Group (Non-Normative)</h2><p>This specification was prepared and approved for publication by the W3C XML Working Group (WG). WG approval of this specification does not necessarily imply that all WG members voted for its approval. The current and former participants of the XML WG are:</p><ul><li>Jon Bosak, Sun (<i>Chair</i>) </li><li>James Clark (<i>Technical Lead</i>) </li><li>Tim Bray, Textuality and Netscape (<i>XML Co-editor</i>) </li><li>Jean Paoli, Microsoft (<i>XML Co-editor</i>) </li><li>C. M. Sperberg-McQueen, U. of Ill. (<i>XML Co-editor</i>) </li><li>Dan Connolly, W3C (<i>W3C Liaison</i>) </li><li>Paula Angerstein, Texcel</li><li>Steve DeRose, INSO</li><li>Dave Hollander, HP</li><li>Eliot Kimber, ISOGEN</li><li>Eve Maler, ArborText</li><li>Tom Magliery, NCSA</li><li>Murray Maloney, SoftQuad, Grif SA, Muzmo and Veo Systems</li><li>MURATA Makoto (FAMILY Given), Fuji Xerox Information Systems</li><li>Joel Nava, Adobe</li><li>Conleth O'Connell, Vignette</li><li>Peter Sharpe, SoftQuad</li><li>John Tigue, DataChannel</li></ul></div><div class="div1"> <h2><a name="sec-core-wg" id="sec-core-wg"/>H W3C XML Core Working Group (Non-Normative)</h2><p>The <span>fifth</span> edition of this specification was prepared by the W3C XML Core Working Group (WG). The participants in the WG at the time of publication of this edition were:</p><ul><li>John Cowan, Google</li><li>Andrew Fang, PTC-Arbortext</li><li>Paul Grosso, PTC-Arbortext (<i>Co-Chair</i>) </li><li>Konrad Lanz, A-SIT</li><li>Glenn Marcy, IBM</li><li>Henry Thompson, W3C (<i>Staff Contact</i>) </li><li>Richard Tobin, University of Edinburgh</li><li>Daniel Veillard</li><li>Norman Walsh, Mark Logic (<i>Co-Chair</i>) </li><li>François Yergeau</li></ul></div><div class="div1"> <h2><a name="prod-notes" id="prod-notes"/>I Production Notes (Non-Normative)</h2><p>This edition was encoded in a slightly modified version of the <a href="https://www.w3.org/2002/xmlspec/dtd/2.10/xmlspec.dtd">XMLspec DTD, v2.10</a>. The XHTML versions were produced with a combination of the <a href="https://www.w3.org/2002/xmlspec/xhtml/1.13/xmlspec.xsl">xmlspec.xsl</a>, <a href="https://www.w3.org/2002/xmlspec/xhtml/1.13/diffspec.xsl">diffspec.xsl</a>, and <a href="REC-xml.xsl">REC-xml.xsl</a> XSLT stylesheets.</p></div><div class="div1"> <h2><a name="sec-suggested-names" id="sec-suggested-names"/>J Suggestions for XML Names (Non-Normative)</h2><p>The following suggestions define what is believed to be best practice in the construction of XML names used as element names, attribute names, processing instruction targets, entity names, notation names, and the values of attributes of type ID, and are intended as guidance for document authors and schema designers. All references to Unicode are understood with respect to a particular version of the Unicode Standard greater than or equal to 5.0; which version should be used is left to the discretion of the document author or schema designer.</p><p>The first two suggestions are directly derived from the rules given for identifiers in Standard Annex #31 (UAX #31) of the Unicode Standard, version 5.0 <a href="#Unicode">[Unicode]</a>, and exclude all control characters, enclosing nonspacing marks, non-decimal numbers, private-use characters, punctuation characters (with the noted exceptions), symbol characters, unassigned codepoints, and white space characters. The other suggestions are mostly derived from Appendix B in previous editions of this specification.</p><ol class="enumar"><li><p>The first character of any name should have a Unicode property of ID_Start, or else be '_' #x5F.</p></li><li><p>Characters other than the first should have a Unicode property of ID_Continue, or be one of the characters listed in the table entitled "Characters for Natural Language Identifiers" in UAX #31, with the exception of "'" #x27 and "’" #x2019.</p></li><li><p>Characters in names should be expressed using Normalization Form C as defined in <a href="#unipart15">[UnicodeNormal]</a>.</p></li><li><p>Ideographic characters which have a canonical decomposition (including those in the ranges [#xF900-#xFAFF] and [#x2F800-#x2FFFD], with 12 exceptions) should not be used in names. </p></li><li><p>Characters which have a compatibility decomposition (those with a "compatibility formatting tag" in field 5 of the Unicode Character Database -- marked by field 5 beginning with a "<") should not be used in names. This suggestion does not apply to characters which despite their compatibility decompositions are in regular use in their scripts, for example #x0E33 THAI CHARACTER SARA AM or #x0EB3 LAO CHARACTER AM.</p></li><li><p>Combining characters meant for use with symbols only (including those in the ranges [#x20D0-#x20EF] and [#x1D165-#x1D1AD]) should not be used in names.</p></li><li><p>The interlinear annotation characters ([#xFFF9-#xFFFB]) should not be used in names.</p></li><li><p>Variation selector characters should not be used in names.</p></li><li><p>Names which are nonsensical, unpronounceable, hard to read, or easily confusable with other names should not be employed.</p></li></ol></div></div><script type="application/javascript" src="https://www.w3.org/scripts/TR/fixup.js"></script></body></html>