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Introduction to XPath

Every XPath expression has two parts: a context node and a node pattern. The context node provides the

context from which the node pattern should begin. The node pattern is a string made up of one or more

node selectors.

For instance, consider the following XML document:

<?xml version=”1.0”?>

<name>Nicholas C. Zakas</name>

</employee>

<name>Jim Smith</name>

</employee>

</employees>

And consider this XPath expression:

employee/name

If the context node is

, then the previous XPath expression matches both

<name>Nicholas

C. Zakas</name>

and

<name>Jim Smith</name>

. In the expression, both

employee

and

name

refer to

tag names of XML elements in the order in which they appear from the context node; the slash indicates a

parent-to-child relationship. In essence, the XPath expression says, “Starting from

, match

any

<name/>

elements located under any

element that is a child of the reference node.”

To select only the first

element’s

<name/>

element, the XPath expression is the following:

employee[position() = 1]/name

In XPath, the square brackets notation is used to provide more specific information about an element.

This example uses the XPath

position()

function, which returns the element’s position under its

parent element. The first child node is in position

, so comparing

position()

matches only the

first

element. Then, the slash and

name

match the

<name/>

element under that first

element.

You can use a variety of ways to match elements in addition to their names and positions. Suppose you

want to select all

elements with the

title

attribute equal to

“Salesperson”

, the XPath

expression would be the following:

employee[@title = “Salesperson”]

In this expression, the

symbol is short for

attribute

XPath is a very powerful expression that can make finding specific nodes within a DOM Document

much easier. Because of this, both IE and Mozilla made sure to include XPath support in their DOM

implementations.

If you’d like to learn more about XPath, consider picking up XPath 2.0: Programmer’s Reference (Wiley

Publishing, Inc., ISBN 0-7645-6910-4).

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