| Table of Content: 
  General overviewThe definitionSimple rules
    
      How to reference a DTD from a documentDeclaring elementsDeclaring attributesSome examplesHow to validateOther resources Well what is validation and what is a DTD ? DTD is the acronym for Document Type Definition. This is a description of
the content for a family of XML files. This is part of the XML 1.0
specification, and allows one to describe and verify that a given document
instance conforms to the set of rules detailing its structure and content. Validation is the process of checking a document against a DTD (more
generally against a set of construction rules). The validation process and building DTDs are the two most difficult parts
of the XML life cycle. Briefly a DTD defines all the possible elements to be
found within your document, what is the formal shape of your document tree
(by defining the allowed content of an element; either text, a regular
expression for the allowed list of children, or mixed content i.e. both text
and children). The DTD also defines the valid attributes for all elements and
the types of those attributes. The W3C XML Recommendation (Tim Bray's annotated version of
Rev1): (unfortunately) all this is inherited from the SGML world, the syntax is
ancient... Writing DTDs can be done in many ways. The rules to build them if you need
something permanent or something which can evolve over time can be radically
different. Really complex DTDs like DocBook ones are flexible but quite
harder to design. I will just focus on DTDs for a formats with a fixed simple
structure. It is just a set of basic rules, and definitely not exhaustive nor
usable for complex DTD design. Assuming the top element of the document is specand the dtd
is placed in the filemydtdin the subdirectorydtdsof the directory from where the document were loaded: <!DOCTYPE spec SYSTEM "dtds/mydtd">
 Notes: 
  The system string is actually an URI-Reference (as defined in RFC 2396) so you can use a
    full URL string indicating the location of your DTD on the Web. This is a
    really good thing to do if you want others to validate your document.It is also possible to associate a PUBLICidentifier (a
    magic string) so that the DTD is looked up in catalogs on the client side
    without having to locate it on the web.A DTD contains a set of element and attribute declarations, but they
    don't define what the root of the document should be. This is explicitly
    told to the parser/validator as the first element of the
    DOCTYPEdeclaration. The following declares an element spec: <!ELEMENT spec (front, body, back?)>
 It also expresses that the spec element contains one front,
onebodyand one optionalbackchildren elements in
this order. The declaration of one element of the structure and its content
are done in a single declaration. Similarly the following declaresdiv1elements: <!ELEMENT div1 (head, (p | list | note)*, div2?)>
 which means div1 contains one headthen a series of optionalp,lists andnotes and then an
optionaldiv2. And last but not least an element can contain
text: <!ELEMENT b (#PCDATA)>
 bcontains text or being of mixed content (text and elements
in no particular order):
 <!ELEMENT p (#PCDATA|a|ul|b|i|em)*>
 p can contain text ora,ul,b,i oremelements in no particular
order.
 Again the attributes declaration includes their content definition: <!ATTLIST termdef name CDATA #IMPLIED>
 means that the element termdefcan have anameattribute containing text (CDATA) and which is optional
(#IMPLIED). The attribute value can also be defined within a
set: <!ATTLIST list type (bullets|ordered|glossary)
"ordered">
 means listelement have atypeattribute with 3
allowed values "bullets", "ordered" or "glossary" and which default to
"ordered" if the attribute is not explicitly specified. The content type of an attribute can be text (CDATA),
anchor/reference/references
(ID/IDREF/IDREFS), entity(ies)
(ENTITY/ENTITIES) or name(s)
(NMTOKEN/NMTOKENS). The following defines that achapterelement can have an optionalidattribute
of typeID, usable for reference from attribute of type
IDREF: <!ATTLIST chapter id ID #IMPLIED>
 The last value of an attribute definition can be #REQUIRED
meaning that the attribute has to be given,#IMPLIEDmeaning that it is optional, or the default value (possibly prefixed by#FIXEDif it is the only allowed). Notes: The directory test/valid/dtds/in the libxml2 distribution
contains some complex DTD examples. The example in the filetest/valid/dia.xmlshows an XML file where the simple DTD is
directly included within the document. The simplest way is to use the xmllint program included with libxml. The
--validoption turns-on validation of the files given as input.
For example the following validates a copy of the first revision of the XML
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 dtdallows validation of the document(s)
against a given DTD. Libxml2 exports an API to handle DTDs and validation, check the associated
description. DTDs are as old as SGML. So there may be a number of examples on-line, I
will just list one for now, others pointers welcome: I suggest looking at the examples found under test/valid/dtd and any of
the large number of books available on XML. The dia example in test/valid
should be both simple and complete enough to allow you to build your own. Daniel Veillard |