XPath function
id()
Selects elements by their DTD-declared ID — which means it does nothing at all in most documents.
id() looks up elements whose ID-typed attribute matches the argument. The catch is what makes an attribute ID-typed: it must be declared so in a DTD, and the DTD must have been read. An attribute merely named 'id' is not an ID.
Because most modern documents carry no DTD, id() usually returns nothing. //*[@id='x'] is what people actually want, and it works everywhere.
- Syntax
id(object) → node-set- Returns
- node-set
- Kind
- Function
- Version
- XPath 1.0 (libxml2)
Worked examples
Each result below is what the evaluator returned, asserted on every test run. An empty result means an empty node-set — which is an answer, not an error.
What catches people out
- An attribute called 'id' is not an ID. Only a DTD (or schema, depending on the processor) can make it one.
- Most XPath tools do not load external DTDs by default, so id() silently returns an empty node-set rather than telling you why.
- Use //*[@id='…'] unless you are genuinely working with a DTD-validated document.
The sample documents
Every example on this page runs against this document. The sitemap one is namespaced on purpose — almost every real XML document is, and that changes which expressions match.
<?xml version="1.0" encoding="UTF-8"?>
<order id="A-1001" tier="gold">
<customer country="NO">Ada Lovelace</customer>
<lines>
<line sku="WIDGET-1" qty="2"><price>14.50</price></line>
<line sku="WIDGET-2" qty="1"><price>39.00</price></line>
<line sku="GIFT-WRAP" qty="1"><price>0.00</price></line>
</lines>
<note> Deliver after 09:00 </note>
</order>Related
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