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XML Pocket Reference, 2nd Edition

Robert Eckstein & Michel Casabianca

Second Edition April 2001

ISBN: 0596001339

XML, the Extensible Markup Language, is the next-generation markup

language for the Web.

It provides a more structured (and therefore more powerful) medium

than HTML, allowing you to define new document types and

stylesheets as needed.

Although the generic tags of HTML are sufficient for everyday text, XML

gives you a way to add rich, well-defined markup to electronic documents.

The XML Pocket Reference is both a handy introduction to XML

terminology and syntax, and a quick reference to XML instructions,

attributes, entities, and datatypes.

Although XML itself is complex, its basic concepts are simple.

This small book combines a perfect tutorial for learning the basics of XML

with a reference to the XML and XSL specifications.

The new edition introduces information on XSLT (Extensible Stylesheet

Language Transformations) and Xpath.

Contents

1.1 Introduction

1.2 XML Terminology

1.3 XML Reference

1.4 Entity and Character References

1.5 Document Type Definitions

1.6 The Extensible Stylesheet Language

1.7 XSLT Stylesheet Structure

1.8 Templates and Patterns

1.9 XSLT Elements

1.10 XPath

1.11 XPointer and XLink

XML Pocket Reference, 2 nd edition

1.1 Introduction

The Extensible Markup Language (XML) is a document-processing standard that is an official

recommendation of the World Wide Web Consortium (W3C), the same group responsible for

overseeing the HTML standard. Many expect XML and its sibling technologies to become the markup

language of choice for dynamically generated content, including nonstatic web pages. Many companies

are already integrating XML support into their products.

XML is actually a simplified form of Standard Generalized Markup Language (SGML), an

international documentation standard that has existed since the 1980s. However, SGML is extremely

complex, especially for the Web. Much of the credit for XML's creation can be attributed to Jon Bosak

of Sun Microsystems, Inc., who started the W3C working group responsible for scaling down SGML to

a form more suitable for the Internet.

Put succinctly, XML is a meta language that allows you to create and format your own document

markups. With HTML, existing markup is static: <HEAD> and <BODY> , for example, are tightly

integrated into the HTML standard and cannot be changed or extended. XML, on the other hand,

allows you to create your own markup tags and configure each to your liking - for example,

<HeadingA> , <Sidebar> , <Quote> , or <ReallyWildFont> . Each of these elements can be defined

through your own document type definitions and stylesheets and applied to one or more XML

documents. XML schemas provide another way to define elements. Thus, it is important to realize that

there are no "correct" tags for an XML document, except those you define yourself.

While many XML applications currently support Cascading Style Sheets (CSS), a more extensible

stylesheet specification exists, called the Extensible Stylesheet Language (XSL). With XSL, you ensure

that XML documents are formatted the same way no matter which application or platform they appear

on.

XSL consists of two parts: XSLT ( transformations ) and XSL-FO ( formatting objects ).

Transformations, as discussed in this book, allow you to work with XSLT and convert XML documents

to other formats such as HTML. Formatting objects are described briefly in Section 1.6.1 .

This book offers a quick overview of XML, as well as some sample applications that allow you to get

started in coding. We won't cover everything about XML. Some XML-related specifications are still in

flux as this book goes to print. However, after reading this book, we hope that the components that

make up XML will seem a little less foreign.

page 1

XML Pocket Reference, 2 nd edition

1.2 XML Terminology

Before we move further, we need to standardize some terminology. An XML document consists of one

or more elements . An element is marked with the following form:

<Body>

This is text formatted according to the Body element

</Body>.

This element consists of two tags : an opening tag, which places the name of the element between a

less-than sign ( < ) and a greater-than sign ( > ), and a closing tag, which is identical except for the

forward slash ( / ) that appears before the element name. Like HTML, the text between the opening and

closing tags is considered part of the element and is processed according to the element's rules.

Elements can have attributes applied, such as the following:

Here, the attribute is specified inside of the opening tag and is called currency . It is given a value of

Euro , which is placed inside quotation marks. Attributes are often used to further refine or modify the

default meaning of an element.

In addition to the standard elements, XML also supports empty elements . An empty element has no

text between the opening and closing tags. Hence, both tags can (optionally) be combined by placing a

forward slash before the closing marker. For example, these elements are identical:

Empty elements are often used to add nontextual content to a document or provide additional

information to the application that parses the XML. Note that while the closing slash may not be used

in single-tag HTML elements, it is mandatory for single-tag XML empty elements.

1.2.1 Unlearning Bad Habits

Whereas HTML browsers often ignore simple errors in documents, XML applications are not nearly as

forgiving. For the HTML reader, there are a few bad habits from which we should dissuade you:

XML is case-sensitive

Element names must be used exactly as they are defined. For example, <Paragraph> and

<paragraph> are not the same.

A non-empty element must have an opening and a closing tag

Each element that specifies an opening tag must have a closing tag that matches it. If it does

not, and it is not an empty element, the XML parser generates an error. In other words, you

cannot do the following:

This is a paragraph.

This is another paragraph.

Instead, you must have an opening and a closing tag for each paragraph element:

<Paragraph>This is another paragraph.</Paragraph>

page 2

<Paragraph>This is a paragraph.</Paragraph>

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