XML Reference Guide

Mar 14, 2003

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In many ways, the heart of XSL Transformations is the ability to choose what elements, attributes, or text to transform. In an XSLT style sheet, you do that using XML Path Language expressions, or XPath.

XPath 1.0, the most commonly supported version at the time of this writing, works mostly on the idea of the XML document as a tree of information using the parent-child relationship between nodes. For example, in the document:

<?xml version="1.0"?>
<order orderid="THX1138" customerNumber="3263827">
    <lineitem itemid="C33">
       <item>3/4" Hex Bolt</item>
       <quantity>36</quantity>
       <unitprice currency="dollars">.35</unitprice>
    </lineitem>
    <lineitem itemid="M48">
       <item>Condenser</item>
       <quantity>1</quantity>
       <unitprice currency="dollars">2200</unitprice>
    </lineitem>
    <delivery>Overnight</delivery>
</order>

You can select the item elements using the expression

/order/lineitem/item

In an XPath expression, the slash (/) is mostly used as an abbreviation of the "child" axis. In other words, we're looking for all item children of the lineitem children of the order children of the document root, represented by the leading slash.

When using XPath, it's easy to be confused about what is actually being selected. For example, a simple XSLT style sheet of

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:template match="/">

    <xsl:value-of select="order/delivery"/>

</xsl:template>

</xsl:stylesheet>

produces a simple result of

<?xml version="1.0" encoding="UTF-8"?>
Overnight

which might give the impression that the expression is selecting the contents of the delivery element, but this is not the case; the expression selects the element node, but the value-of element outputs only the contents of the element, giving the misleading results. The copy-of element gives a little better idea of what's actually being selected. The style sheet

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:template match="/">

           <xsl:copy-of select="order/delivery/text()"/><xsl:text>

</xsl:text><xsl:copy-of select="order/delivery"/>

</xsl:template>

</xsl:stylesheet>

provides a result of

<?xml version="1.0" encoding="UTF-8"?>
Overnight

<delivery>Overnight</delivery>

While the child axis is the most commonly used, it is not the only one available. Axes such as parent or descendants-or-self provide a great deal of flexibility in navigating the tree. Perhaps the second most commonly used axis is the attribute axis, abbreviated as @. To select the customerNumber attribute, for example, you could us the expression

order/@customerNumber

The attribute axis is frequently used in a predicate, which is a way of narrowing down a result even further. For example, the expression

order/lineitem[@itemid='M48']

selects only the lineitem element with an itemid attribute equal to M48.

In many ways, XPath is to XML what regular expressions are to text processing; they provide a way to find just about anything within a document, as long as you know what you're looking for. XPath is also at the heart of related specifications such as XLink and XPointer, and is used to lesser and greater degrees in other specifications.

Because of its importance, it should not come as a surprise that XPath 2.0 is now in the works, in tandem with XML Query language, which provides a more data-oriented way of looking at XML.

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