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</style><title>Validation & DTDs</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000">
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Validation & DTDs
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Table of Content:
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  1. General overview
    		2.
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  2. The definition
    		3.
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  3. Simple rules
    		4.
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    1. How to reference a DTD from a document
      		2.
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    2. Declaring elements
      		3.
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    3. Declaring attributes
      		4.
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    4. Some examples
      		5.
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    5. How to validate
      		6.
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    6. Other resources
      		7.
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      General overview
      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 definition
      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...
        Simple rules
        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.
        How to reference a DTD from a document:
        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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          Declaring elements:
          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, lists and notes 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.
          Declaring attributes:
          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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            Some examples
            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.
            How to validate
            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.
            Other resources
            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
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