XSLT Reference
generate-id()
Returns a unique string identifier for a node, guaranteed to be a valid XML name and stable within a single transformation run.
generate-id(node?)Description
generate-id() returns a string that uniquely identifies a node within the current transformation. The string is guaranteed to:
- Be a valid XML
Name(it can be used as an attribute value or part of an ID). - Be unique: different nodes in the same transformation produce different IDs.
- Be consistent: calling
generate-id()on the same node multiple times within one transformation always returns the same string.
The generated value is arbitrary and processor-specific. It may change between runs or between different XSLT processors; do not store it in output that must be reproducible.
When called with no argument, generate-id() uses the context node. If the argument is an empty node-set, the empty string "" is returned.
Common uses include generating id/href pairs for internal cross-references in HTML output, implementing the Muenchian grouping technique, and creating unique element names when converting to formats that require them.
Parameters
| Parameter | Type | Required | Description |
|---|---|---|---|
node | node-set | No | The node to identify. Defaults to the context node. If empty, returns "". |
Return value
xs:string — a unique, valid XML Name for the node, or "" for an empty node-set.
Examples
Generate anchor links in HTML output
Input XML:
<?xml version="1.0" encoding="UTF-8"?>
<sections>
<section><title>Introduction</title><p>Welcome.</p></section>
<section><title>Usage</title><p>How to use it.</p></section>
</sections>
Stylesheet:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" indent="yes"/>
<xsl:template match="/sections">
<html><body>
<!-- Table of contents -->
<ul>
<xsl:for-each select="section">
<li><a href="#{generate-id()}"><xsl:value-of select="title"/></a></li>
</xsl:for-each>
</ul>
<!-- Content -->
<xsl:for-each select="section">
<h2 id="{generate-id()}"><xsl:value-of select="title"/></h2>
<p><xsl:value-of select="p"/></p>
</xsl:for-each>
</body></html>
</xsl:template>
</xsl:stylesheet>
Output (IDs are processor-generated):
<html><body>
<ul>
<li><a href="#d0e2">Introduction</a></li>
<li><a href="#d0e7">Usage</a></li>
</ul>
<h2 id="d0e2">Introduction</h2>
<p>Welcome.</p>
<h2 id="d0e7">Usage</h2>
<p>How to use it.</p>
</body></html>
Muenchian grouping — test node identity
Input XML:
<?xml version="1.0" encoding="UTF-8"?>
<items>
<item cat="A">One</item>
<item cat="B">Two</item>
<item cat="A">Three</item>
</items>
Stylesheet:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:key name="by-cat" match="item" use="@cat"/>
<xsl:template match="/items">
<groups>
<xsl:for-each select="item[generate-id() = generate-id(key('by-cat', @cat)[1])]">
<group cat="{@cat}">
<xsl:value-of select="count(key('by-cat', @cat))"/> items
</group>
</xsl:for-each>
</groups>
</xsl:template>
</xsl:stylesheet>
Output:
<groups>
<group cat="A">2 items</group>
<group cat="B">1 items</group>
</groups>
Notes
- The generated ID is not persistent: it may differ between processors and between runs of the same processor. Do not use it as a stable key in output databases or cross-document references.
generate-id()on an empty node-set returns"", not an error. Check for this case if the argument might be empty.- Two calls to
generate-id()on the same node within one transformation always return the same value, which is the property that makes the Muenchian grouping technique work. - In XSLT 2.0+,
generate-id()remains available and unchanged. Thexsl:for-each-groupinstruction is usually a cleaner alternative for grouping tasks.