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XML Terminology

To understand the nature of XML Schema it is important to be familiar with some basic XML terminology.

An XML tag begins with a '<' and ends with a '>'. For example:

    <foo>

The XML element shown below comprises a start tag ('<name>'), some character content ('Mithrandir'), and an end tag ('</name>').

    <name>Mithrandir</name>

In additional to character content, an element may include other child elements nested within in. In this example, the '<wizard> ... </wizard>' element contains the '<name> ... </name>' and '<alias> ... </alias>' elements.

    <wizard>
      <name>Mithrandir</name>
      <alias>Gandalf</alias>
    </wizard>

The start tag of an element may contain attributes each comprising of a name and quoted value.

    <name lang="Elvish">Mithrandir</name>

An element is said to have a simple type if it contains character content, the whole character content and nothing but the character content.

    <name>Mithrandir</nam>	    # simple type

If an element has attributes or non-character child elements then it is said to have a complex type.

    <name lang="EN">Frank</name>    # complex type (attributes)

    <name>			    # complex type (child elements)
      <first>Frank</first>
      <last>Sidebottom</last>
    </name>

A complex element defines a content model to specify what is permitted within the element in terms of character content and child elements. Content models may be defined in terms of particles which specify the child elements that may appear within an element, along with minimum and maximum occurence constraints. These particles may be specified as a sequence (each element particle must match in order), a choice (match just one particle) or all (match all particles but in any order). Particles can be nested recursively allowing content models of arbitrary complexity to be defined.

    <wizard>			    # sequence:
      <name>Gandalf</name>	    #   name   min=1 max=1
      <colour>Grey</colour>	    #   colour min=1 max=1
      <alias>Mithrandir</alias>     #   alias  min=0 max=unbounded
      <alias>Beardy</alias>
      ...
    </wizard>

If an element accepts both character content and child elements then it is said to have a mixed content model.

    <p>
      An HTML paragraph is an example of an XML element with a 
      <b>mixed content model</b>.  It can contain plain text 
      and <i>other XML elements</i>
    </p>

Elements can also have an empty content model in which case the start and end tags can be combined like this:

    <br />

NOTE: The space before the trailing '/' is optional. However, if you find yourself writing XHTML documents, then you should always add the space to allow HTML browsers to display the markup correctly.

XML Schema Overview

A schema is a description of the permitted structure and characteristics of a class of XML instance documents. W3C XML Schema is one particular schema standard and is documented extensively at http://w3c.org/.

This is an XML fragment which describes a <person>. This is termed an "XML instance document".

    <person id="abw">
      <name>
        <first>Andy</first> <last>Wardley</last>
      </name>
     <email>abw@kfs.org</email>
   </person>

Here is the schema for this element:

    <schema>

      <element name="person" type="personType"/>

      <complexType name="personType">
        <attribute name="id" type="string"/>
        <element name="name" type="nameType"/>
        <element name="email" type="emailType"/>
      </complexType>

      <complexType name="nameType">
        <element name="first" type="string"/>
        <element name="last" type="string"/>
      </complexType>

      <simpleType name="emailType">
        <restriction base="string">
          <pattern value="\w+@(\w+\.)+\w+"/>
        </restriction>
      </simpleType>

    </schema>

XML Schema Perl Modules

These Perl modules implement various objects which can be used to define schemata. At the time of writing, these modules offer "minimal conformance" but not "full conformance". Full conformance implies that a schema can be specified as an XML document (like that above) which can be processed automatically to construct an internal schema representation. Currently, schemata must be built "by hand" as Perl programs as shown below. However, we intend to encode the schema for XML Schema itself to build a minimally conformant parser which can bootstrap itself into being a fully conformant parser.

    use XML::Schema;

    my $schema = XML::Schema->new();
 
    #------------------------------------------------------------
    # define simple type for email addresses

    my $emailType = $schema->simpleType( name => 'email',
				         base => 'string' );

    $emailType->constrain( pattern => '\w+@(\w+\.)+\w+' );

    #------------------------------------------------------------
    # define complex type for name

    my $nameType = $schema->complexType( name => 'nameType' );

    $nameType->element( name => 'first', 
			type => 'string' );

    $nameType->element( name => 'last', 
			type => 'string' );

    #------------------------------------------------------------
    # define complex type for person

    my $personType = $schema->complexType( name => 'personType' );

    $personType->attribute( name => 'id', 
			    type => 'string' );

    $personType->element( name => 'name', 
			  type => 'nameType' );

    $personType->element( name => 'email', 
			  type => 'emailType' );

    #------------------------------------------------------------
    # define key schema element

    $schema->element( name => 'person',
		      type => 'personType' );

Having constructed a schema model in this way, an XML parser can be generated to parse instances of this schema.

    my $parser = $schema->parser();

    my $result = $parser->parse('person.xml');