Table of Content:
- General overview
- The definition
- Simple rules
- How to reference a DTD from a document
- Declaring elements
- Declaring attributes
- Some examples
- How to validate
- Other 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
(unfortunately) all this is inherited from the SGML world, the syntax is
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
spec and the dtd
is placed in the file
mydtd in the subdirectory
dtds of the directory from where the document were loaded:
<!DOCTYPE spec SYSTEM "dtds/mydtd">
- 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
PUBLIC identifier (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
The following declares an element
<!ELEMENT spec (front, body, back?)>
It also expresses that the spec element contains one
body and one optional
back children elements in
this order. The declaration of one element of the structure and its content
are done in a single declaration. Similarly the following declares
<!ELEMENT div1 (head, (p | list | note)*, div2?)>
which means div1 contains one
head then a series of optional
notes and then an
div2. And last but not least an element can contain
<!ELEMENT b (#PCDATA)>
b contains 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 or
em elements in no particular
Again the attributes declaration includes their content definition:
<!ATTLIST termdef name CDATA #IMPLIED>
means that the element
termdef can have a
attribute containing text (
CDATA) and which is optional
#IMPLIED). The attribute value can also be defined within a
<!ATTLIST list type (bullets|ordered|glossary)
list element have a
type attribute 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 (
ENTITIES) or name(s)
NMTOKENS). The following defines that a
chapter element can have an optional
ID, usable for reference from attribute of type
<!ATTLIST chapter id ID #IMPLIED>
The last value of an attribute definition can be
meaning that the attribute has to be given,
meaning that it is optional, or the default value (possibly prefixed by
#FIXED if it is the only allowed).
test/valid/dtds/ in the libxml2 distribution
contains some complex DTD examples. The example in the file
test/valid/dia.xml shows 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
--valid option turns-on validation of the files given as input.
For example the following validates a copy of the first revision of the XML
xmllint --valid --noout test/valid/REC-xml-19980210.xml
the -- noout is used to disable output of the resulting tree.
--dtdvalid dtd allows validation of the document(s)
against a given DTD.
Libxml2 exports an API to handle DTDs and validation, check the associated
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.