XML element has namespace, my XPATH does not work - xslt

Question

I was given the following XML:
<root>
<items>
<item>
<title>Item</title>
<details>
<data xmlns="http://some_url">
<length>10</length>
<weight>1.2</weight>
</data>
</details>
</item>
</items>
</root>
Following XPath does not work meaning nothing is printed like the "data" element does not exists:
/root/items/item/details/data
But when I remove "xmlns" namespace attribute of "data" element it's content is printed.
How should the xpath expression look like to work without deleting "xmlns" namespace attribute of "data" element?
I'm using SAXON and XSL 1.0.

Answer 1

This is one of the most FAQ in XPath / XSLT:
XPath interprets an unprefixed element name as belonging to "no namespace" and this is the reason elements with unprefixed names belonging to a default (nonempty) namespace aren't selected when only their unprefixed name is specified as a node-test in an XPath expression.
The solution is either:
Create a namespace binding where a prefix (say "x") is associated with the default namespace, then specify x:elementName instead of elementName.
Use long, ugly and unreliable expressions like: *[name() = 'elementName']
Here is an XSLT transformation using the above method1. :
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:a="http://some_url">
<xsl:output method="text"/>
<xsl:template match="/">
<xsl:value-of select=
"/root/items/item/details/a:data/a:weight"/>
</xsl:template>
</xsl:stylesheet>
When this transformation is applied (using Saxon 6.5.4 or any other compliant XSLT 1.0 processor) on the provided XML document:
<root>
<items>
<item>
<title>Item</title>
<details>
<data xmlns="http://some_url">
<length>10</length>
<weight>1.2</weight>
</data>
</details>
</item>
</items>
</root>
The correct/wanted node is selected and its string value is copied to the output:
1.2

Answer 2

In XPath, you have to assign a prefix to the namespace. How you do that depends on the XPath software/library you are using, but assuming you associate the namespace URI http://some_url with the namespace prefix someUrl, you can change your XPath expression as follows:
/root/items/item/details/someUrl:data