<xsl:stylesheet ... > 
<xsl:template  match =" * " >
  <xsl:copy>
    <xsl:apply-templates />
  </xsl:copy>
</xsl:template>
<xsl:template match=" thought " >
  <xsl:copy>
    <title><xsl:value-of  select =" title " /><title>
  <xsl:apply-templates  select =" author " />
  <body><xsl:value-of select=" body " /><body>
</xsl:copy>
</xsl:template>
<xsl:template match=" author " >
  <xsl:copy>
   <xsl:text>The author is: </xsl:text>
   <xsl:value-of select=" . " />
  </xsl:copy>
</xsl:template>
</xsl:stylesheet>

XPath

XSLT is crucially dependent on XPath for many functions
XPath defines patterns for the match attribute of xsl:template:
```
<xsl:template match="thought/author" >
```
XPath defines expressions for the select attribute of xsl:apply-templates:
```
<xsl:apply-templates select="author" />
```
XPath defines expressions for the select attribute of xsl:value-of:
```
<creator><xsl:value-of select="author" />
```
We will see that many template instructions also use XPath
XPath expressions can yield values of type:
- Node-set (collection of nodes)
- Boolean value
- Number
- String

Expression evaluation often occurs with respect to a context node

<xsl:template match="author">
..... context node is the author node being processed
<xsl:value-of select="."/>  Selects the contents of the context node
<xsl:value-of select="@xyz"/>  The xyz attribute of the context node
</xsl:template>

Example

An example to show what XPath can do
A hierarchy of nodes where each element either contains other elements or is empty
There are no text nodes

<aaa><bbb><bbb><bbb><aa><aa><a /><g /><h /><h /></aa></aa></bbb>
</bbb></bbb>
<ccc><ccc /><ddd><ccc><aa /><bb><aa><h /><c /><e /><h /><e /><e />
<h /></aa><cc /><cc><g /><g /><f /><e /><f /></cc>
</bb><bb /></ccc><ccc /><ddd /></ddd><ccc /></ccc><ddd><ddd /></ddd>
<ccc><bbb /><ccc><ddd><cc /><cc><cc /><cc><h /><h /><e /><f /></cc>
<bb /><dd /></cc><dd /><dd /><ee><dd><f /><f /><f /><d /><g /><c /></dd>
</ee><ee /><ee /></ddd><ddd />
<eee><ff /><ff><ee /><ee><e /><e /><h /><f /><c /><h /><c /><d /></ee>
<ee /><dd><b /><c /><d /><d /><d /><d /></dd>
<cc /><aa /><bb /></ff><ee /><ee /><dd /></eee><ddd />
</ccc><eee /></ccc>
<bbb><bbb><eee><cc><bb><g /><e /><e><h /><h />
<f /><f /><c /><f /><f /><h /></e><f /><c />
</bb></cc></eee></bbb></bbb></aaa>

You can search up and down the tree for nodes to change using various axes:

Starting from the highlighted cc node, something like this will select just the two c nodes at the bottom. XPath is a very powerful system for just changing the things that need to be changed.

XPath expressions get used for a variety of purposes in XSLT:

XPath is a powerful tool for selecting which nodes to operate
Used in four ways:
1. match attribute determines which nodes a template applies to
2. test attribute determines whether a condition applies
3. select attribute selects a set of nodes for processing
4. select attribute selects a value for processing
XPath and XSLT functions defined to aid XPath in these activities

XPath Expressions

Navigation

XPath

Example