XML – A Quick Introduction

Kerry Raymond(stolen from others)

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What is XML?

• XML = Extensible Mark-up Language

• XML is a simplified version of SGML (Standard Generalized Mark-up Language)

• Standardised by W3C

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Example XML

<?xml version=“1.0” standalone=“yes” ?><bibliography> <bibItem id=“horgan96” type=“book”> <title>The End of Science</title> <author>John Horgan</author> <date><year>1996</year></date> <publisher>Little, Brown and Company</publisher> </bibItem>

<!-- More bibItem elements. -->

</bibliography>

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XML mark up

• The main parts of an XML document are:– Tag– Element– Attribute

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XML mark up - Tag

• Tags are the most familiar aspect of XML mark up.

• Kinds of tags:– Start tag: <title>.– End tag: </title>. Start and end tags form

pairs, with some content between them.– Empty tags (no content): <newLine/>.

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XML mark up - Element

• The portion of a document between an opening tag and its corresponding closing tag.

• Example:– <title>The End of Science</title>

• May be empty or have nested content.

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XML mark up - Attribute

• Attributes belong to the element, rather than to the content of the element.

• For example, the attribute “id”:– <bibItem id=“horgan96”>

• Attributes are typed. For example:– CDATA (string).– Enumeration (one of a list of values)– ID (a unique identifier)– IDREF (reference to an ID – simple cross-

reference)

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XML example

<?xml version=“1.0” standalone=“yes” ?><bibliography> <bibItem id=“horgan96” type=“book”> <title>The End of Science</title> <author>John Horgan</author> <date><year>1996</year></date> <publisher>Little, Brown and Company</publisher> </bibItem>

<!-- More bibItem elements. -->

</bibliography>

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Well-formed vs. valid

XML has two levels of acceptability:• Well-formed documents.

– Minimal level required by XML

– Syntactically correct and opening and closing tags are properly nested.

• Valid documents.– Document must be well-formed and must satisfy

the Document Type Definition in all details.

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DTD

• A document type is defined by a DTD. The DTD defines:

• The tree structure a document of a given type can form.– The positions of elements, attributes and so on in

document instances of that type.

– The relationships between elements.

• Any constraints on each of the elements and attributes.

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DTD – Example<!DOCTYPE bibliography [<!ELEMENT bibliography (bibItem)+><!ELEMENT bibItem (title, (author | editor)*, date, publisher?)><!ELEMENT title (#PCDATA)><!ELEMENT author (#PCDATA)><!ELEMENT editor (#PCDATA)><!ELEMENT date (year, month?)><!ELEMENT year (#PCDATA)><!ELEMENT month (#PCDATA)><!ELEMENT publisher (#PCDATA)><!ATTLIST bibItem id ID #REQUIRED type (article | book | report | video | audio) 'article'>]>

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So is it valid?

<?xml version=“1.0” standalone=“yes” ?><!DOCTYPE bibliography SYSTEM “biblio.dtd”><bibliography> <bibItem id=“horgan96” type=“book”> <title>The End of Science</title> <author>John Horgan</author> <date><year>1996</year></date> <publisher>Little, Brown and Company</publisher> </bibItem>

<!-- More bibItem elements. -->

</bibliography>