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</style><title>Validation & DTDs</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"> | | The XML C parser and toolkit of GnomeValidation & DTDs |
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<center>Main Menu</center> | <form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form> |
<center>Related links</center> | |
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General overview
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The definition
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Simple rules
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How to reference a DTD from a document
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Declaring elements
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Declaring attributes
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Some examples
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How to validate
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Other resources
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Well what is validation and what is a DTD ? DTD is the acronym for Document Type Definition. This is a description of
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the content for a family of XML files. This is part of the XML 1.0
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specification, and allows one to describe and verify that a given document
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instance conforms to the set of rules detailing its structure and content.Validation is the process of checking a document against a DTD (more
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generally against a set of construction rules).The validation process and building DTDs are the two most difficult parts
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of the XML life cycle. Briefly a DTD defines all the possible elements to be
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found within your document, what is the formal shape of your document tree
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(by defining the allowed content of an element; either text, a regular
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expression for the allowed list of children, or mixed content i.e. both text
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and children). The DTD also defines the valid attributes for all elements and
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the types of those attributes.The W3C XML Recommendation (Tim Bray's annotated version of
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Rev1):
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Declaring
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elements
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Declaring
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attributes
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(unfortunately) all this is inherited from the SGML world, the syntax is
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ancient...Writing DTDs can be done in many ways. The rules to build them if you need
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something permanent or something which can evolve over time can be radically
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different. Really complex DTDs like DocBook ones are flexible but quite
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harder to design. I will just focus on DTDs for a formats with a fixed simple
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structure. It is just a set of basic rules, and definitely not exhaustive nor
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usable for complex DTD design.Assuming the top element of the document is spec and the dtd
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is placed in the file mydtd in the subdirectory
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dtds of the directory from where the document were loaded:<!DOCTYPE spec SYSTEM "dtds/mydtd">
Notes:
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The system string is actually an URI-Reference (as defined in RFC 2396) so you can use a
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full URL string indicating the location of your DTD on the Web. This is a
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really good thing to do if you want others to validate your document.
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It is also possible to associate a PUBLIC identifier (a
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magic string) so that the DTD is looked up in catalogs on the client side
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without having to locate it on the web.
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A DTD contains a set of element and attribute declarations, but they
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don't define what the root of the document should be. This is explicitly
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told to the parser/validator as the first element of the
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DOCTYPE declaration.
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The following declares an element spec : <!ELEMENT spec (front, body, back?)>
It also expresses that the spec element contains one front ,
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one body and one optional back children elements in
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this order. The declaration of one element of the structure and its content
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are done in a single declaration. Similarly the following declares
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div1 elements:<!ELEMENT div1 (head, (p | list | note)*, div2?)>
which means div1 contains one head then a series of optional
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p , list s and note s and then an
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optional div2 . And last but not least an element can contain
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text:<!ELEMENT b (#PCDATA)>
b contains text or being of mixed content (text and elements
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in no particular order):<!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
p can contain text or a , ul ,
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b , i or em elements in no particular
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order.Again the attributes declaration includes their content definition: <!ATTLIST termdef name CDATA #IMPLIED>
means that the element termdef can have a name
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attribute containing text (CDATA ) and which is optional
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(#IMPLIED ). The attribute value can also be defined within a
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set:<!ATTLIST list type (bullets|ordered|glossary)
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"ordered">means list element have a type attribute with 3
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allowed values "bullets", "ordered" or "glossary" and which default to
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"ordered" if the attribute is not explicitly specified.The content type of an attribute can be text (CDATA ),
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anchor/reference/references
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(ID /IDREF /IDREFS ), entity(ies)
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(ENTITY /ENTITIES ) or name(s)
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(NMTOKEN /NMTOKENS ). The following defines that a
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chapter element can have an optional id attribute
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of type ID , usable for reference from attribute of type
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IDREF:<!ATTLIST chapter id ID #IMPLIED>
The last value of an attribute definition can be #REQUIRED
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meaning that the attribute has to be given, #IMPLIED
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meaning that it is optional, or the default value (possibly prefixed by
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#FIXED if it is the only allowed).Notes:
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Usually the attributes pertaining to a given element are declared in a
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single expression, but it is just a convention adopted by a lot of DTD
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writers:
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<!ATTLIST termdef
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id ID #REQUIRED
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name CDATA #IMPLIED>
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The previous construct defines both id and
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name attributes for the element termdef .
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The directory test/valid/dtds/ in the libxml2 distribution
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contains some complex DTD examples. The example in the file
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test/valid/dia.xml shows an XML file where the simple DTD is
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directly included within the document.The simplest way is to use the xmllint program included with libxml. The
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--valid option turns-on validation of the files given as input.
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For example the following validates a copy of the first revision of the XML
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1.0 specification:xmllint --valid --noout test/valid/REC-xml-19980210.xml
the -- noout is used to disable output of the resulting tree. The --dtdvalid dtd allows validation of the document(s)
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against a given DTD.Libxml2 exports an API to handle DTDs and validation, check the associated
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description.DTDs are as old as SGML. So there may be a number of examples on-line, I
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will just list one for now, others pointers welcome:
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XML-101 DTD
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I suggest looking at the examples found under test/valid/dtd and any of
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the large number of books available on XML. The dia example in test/valid
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should be both simple and complete enough to allow you to build your own.Daniel Veillard </body></html>
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