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1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4 TD {font-family: Verdana,Arial,Helvetica} 5 BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6 H1 {font-family: Verdana,Arial,Helvetica} 7 H2 {font-family: Verdana,Arial,Helvetica} 8 H3 {font-family: Verdana,Arial,Helvetica} 9 A:link, A:visited, A:active { text-decoration: underline } 10 </style><title>Validation & DTDs</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software 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cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Table of Content:</p><ol> 11 <li><a href="#General5">General overview</a></li> 12 <li><a href="#definition">The definition</a></li> 13 <li><a href="#Simple">Simple rules</a> 14 <ol> 15 <li><a href="#reference">How to reference a DTD from a document</a></li> 16 <li><a href="#Declaring">Declaring elements</a></li> 17 <li><a href="#Declaring1">Declaring attributes</a></li> 18 </ol> 19 </li> 20 <li><a href="#Some">Some examples</a></li> 21 <li><a href="#validate">How to validate</a></li> 22 <li><a href="#Other">Other resources</a></li> 23 </ol><h3><a name="General5" id="General5">General overview</a></h3><p>Well what is validation and what is a DTD ?</p><p>DTD is the acronym for Document Type Definition. This is a description of 24 the content for a family of XML files. This is part of the XML 1.0 25 specification, and allows one to describe and verify that a given document 26 instance conforms to the set of rules detailing its structure and content.</p><p>Validation is the process of checking a document against a DTD (more 27 generally against a set of construction rules).</p><p>The validation process and building DTDs are the two most difficult parts 28 of the XML life cycle. Briefly a DTD defines all the possible elements to be 29 found within your document, what is the formal shape of your document tree 30 (by defining the allowed content of an element; either text, a regular 31 expression for the allowed list of children, or mixed content i.e. both text 32 and children). The DTD also defines the valid attributes for all elements and 33 the types of those attributes.</p><h3><a name="definition1" id="definition1">The definition</a></h3><p>The <a href="http://www.w3.org/TR/REC-xml">W3C XML Recommendation</a> (<a href="http://www.xml.com/axml/axml.html">Tim Bray's annotated version of 34 Rev1</a>):</p><ul> 35 <li><a href="http://www.w3.org/TR/REC-xml#elemdecls">Declaring 36 elements</a></li> 37 <li><a href="http://www.w3.org/TR/REC-xml#attdecls">Declaring 38 attributes</a></li> 39 </ul><p>(unfortunately) all this is inherited from the SGML world, the syntax is 40 ancient...</p><h3><a name="Simple1" id="Simple1">Simple rules</a></h3><p>Writing DTDs can be done in many ways. The rules to build them if you need 41 something permanent or something which can evolve over time can be radically 42 different. Really complex DTDs like DocBook ones are flexible but quite 43 harder to design. I will just focus on DTDs for a formats with a fixed simple 44 structure. It is just a set of basic rules, and definitely not exhaustive nor 45 usable for complex DTD design.</p><h4><a name="reference1" id="reference1">How to reference a DTD from a document</a>:</h4><p>Assuming the top element of the document is <code>spec</code> and the dtd 46 is placed in the file <code>mydtd</code> in the subdirectory 47 <code>dtds</code> of the directory from where the document were loaded:</p><p><code><!DOCTYPE spec SYSTEM "dtds/mydtd"></code></p><p>Notes:</p><ul> 48 <li>The system string is actually an URI-Reference (as defined in <a href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396</a>) so you can use a 49 full URL string indicating the location of your DTD on the Web. This is a 50 really good thing to do if you want others to validate your document.</li> 51 <li>It is also possible to associate a <code>PUBLIC</code> identifier (a 52 magic string) so that the DTD is looked up in catalogs on the client side 53 without having to locate it on the web.</li> 54 <li>A DTD contains a set of element and attribute declarations, but they 55 don't define what the root of the document should be. This is explicitly 56 told to the parser/validator as the first element of the 57 <code>DOCTYPE</code> declaration.</li> 58 </ul><h4><a name="Declaring2" id="Declaring2">Declaring elements</a>:</h4><p>The following declares an element <code>spec</code>:</p><p><code><!ELEMENT spec (front, body, back?)></code></p><p>It also expresses that the spec element contains one <code>front</code>, 59 one <code>body</code> and one optional <code>back</code> children elements in 60 this order. The declaration of one element of the structure and its content 61 are done in a single declaration. Similarly the following declares 62 <code>div1</code> elements:</p><p><code><!ELEMENT div1 (head, (p | list | note)*, div2?)></code></p><p>which means div1 contains one <code>head</code> then a series of optional 63 <code>p</code>, <code>list</code>s and <code>note</code>s and then an 64 optional <code>div2</code>. And last but not least an element can contain 65 text:</p><p><code><!ELEMENT b (#PCDATA)></code></p><p><code>b</code> contains text or being of mixed content (text and elements 66 in no particular order):</p><p><code><!ELEMENT p (#PCDATA|a|ul|b|i|em)*></code></p><p><code>p </code>can contain text or <code>a</code>, <code>ul</code>, 67 <code>b</code>, <code>i </code>or <code>em</code> elements in no particular 68 order.</p><h4><a name="Declaring1" id="Declaring1">Declaring attributes</a>:</h4><p>Again the attributes declaration includes their content definition:</p><p><code><!ATTLIST termdef name CDATA #IMPLIED></code></p><p>means that the element <code>termdef</code> can have a <code>name</code> 69 attribute containing text (<code>CDATA</code>) and which is optional 70 (<code>#IMPLIED</code>). The attribute value can also be defined within a 71 set:</p><p><code><!ATTLIST list type (bullets|ordered|glossary) 72 "ordered"></code></p><p>means <code>list</code> element have a <code>type</code> attribute with 3 73 allowed values "bullets", "ordered" or "glossary" and which default to 74 "ordered" if the attribute is not explicitly specified.</p><p>The content type of an attribute can be text (<code>CDATA</code>), 75 anchor/reference/references 76 (<code>ID</code>/<code>IDREF</code>/<code>IDREFS</code>), entity(ies) 77 (<code>ENTITY</code>/<code>ENTITIES</code>) or name(s) 78 (<code>NMTOKEN</code>/<code>NMTOKENS</code>). The following defines that a 79 <code>chapter</code> element can have an optional <code>id</code> attribute 80 of type <code>ID</code>, usable for reference from attribute of type 81 IDREF:</p><p><code><!ATTLIST chapter id ID #IMPLIED></code></p><p>The last value of an attribute definition can be <code>#REQUIRED 82 </code>meaning that the attribute has to be given, <code>#IMPLIED</code> 83 meaning that it is optional, or the default value (possibly prefixed by 84 <code>#FIXED</code> if it is the only allowed).</p><p>Notes:</p><ul> 85 <li>Usually the attributes pertaining to a given element are declared in a 86 single expression, but it is just a convention adopted by a lot of DTD 87 writers: 88 <pre><!ATTLIST termdef 89 id ID #REQUIRED 90 name CDATA #IMPLIED></pre> 91 <p>The previous construct defines both <code>id</code> and 92 <code>name</code> attributes for the element <code>termdef</code>.</p> 93 </li> 94 </ul><h3><a name="Some1" id="Some1">Some examples</a></h3><p>The directory <code>test/valid/dtds/</code> in the libxml2 distribution 95 contains some complex DTD examples. The example in the file 96 <code>test/valid/dia.xml</code> shows an XML file where the simple DTD is 97 directly included within the document.</p><h3><a name="validate1" id="validate1">How to validate</a></h3><p>The simplest way is to use the xmllint program included with libxml. The 98 <code>--valid</code> option turns-on validation of the files given as input. 99 For example the following validates a copy of the first revision of the XML 100 1.0 specification:</p><p><code>xmllint --valid --noout test/valid/REC-xml-19980210.xml</code></p><p>the -- noout is used to disable output of the resulting tree.</p><p>The <code>--dtdvalid dtd</code> allows validation of the document(s) 101 against a given DTD.</p><p>Libxml2 exports an API to handle DTDs and validation, check the <a href="http://xmlsoft.org/html/libxml-valid.html">associated 102 description</a>.</p><h3><a name="Other1" id="Other1">Other resources</a></h3><p>DTDs are as old as SGML. So there may be a number of examples on-line, I 103 will just list one for now, others pointers welcome:</p><ul> 104 <li><a href="http://www.xml101.com:8081/dtd/">XML-101 DTD</a></li> 105 </ul><p>I suggest looking at the examples found under test/valid/dtd and any of 106 the large number of books available on XML. The dia example in test/valid 107 should be both simple and complete enough to allow you to build your own.</p><p></p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>