Why is this xmlns attribute messing up my xpath query?

Question

Why is this xmlns attribute messing up my xpath query?

731 Views Asked by jackthehipster At 11 November 2014 at 15:56

I'm parsing a simple jhove output using LibXML. However, I don't get the values I expect. Here's the code:

use feature "say";
use XML::LibXML;

my $PRSR = XML::LibXML->new();
my $xs=<DATA>; 
say $xs;
my $t1 = $PRSR->load_xml(string => $xs);
say "1:" . $t1->findvalue('//date');
$xs=<DATA>; 
say $xs;
$t1 = $PRSR->load_xml(string => $xs);
say "2:" . $t1->findvalue('//date');


__DATA__
<jhove xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://hul.harvard.edu/ois/xml/ns/jhove" xsi:schemaLocation="http://hul.harvard.edu/ois/xml/ns/jhove http://hul.harvard.edu/ois/xml/xsd/jhove/1.3/jhove.xsd" name="Jhove" release="1.0 (beta 3)" date="2005-02-04"><date>2006-10-06T09:11:34+02:00</date></jhove>
<jhove><date>2006-10-06T09:11:34+02:00</date></jhove>

As you can see, the line "1:" is returning an empty string, while "2:" is returning the expected date. What is in the jhove-root-element that keeps the xpath query from working properly? I even tried in XML-Spy and there it works, even with the full header.

Edit: When I remove the xmlns-attribute from the root element, the xpath query works. But how is that possible?

Original Q&A

There are 2 best solutions below

**tobyink** · Answer 1 · 2014-11-11T16:25:41.883000

The XML::LibXML::Node documentation specifically mentions this issue and how to deal with it...

NOTE ON NAMESPACES AND XPATH:

A common mistake about XPath is to assume that node tests consisting of an element name with no prefix match elements in the default namespace. This assumption is wrong - by XPath specification, such node tests can only match elements that are in no (i.e. null) namespace.

So, for example, one cannot match the root element of an XHTML document with $node->find('/html') since '/html' would only match if the root element <html> had no namespace, but all XHTML elements belong to the namespace http://www.w3.org/1999/xhtml. (Note that xmlns="..." namespace declarations can also be specified in a DTD, which makes the situation even worse, since the XML document looks as if there was no default namespace).

There are several possible ways to deal with namespaces in XPath:
The recommended way is to use the XML::LibXML::XPathContext module to define an explicit context for XPath evaluation, in which a document independent prefix-to-namespace mapping can be defined. For example:
my $xpc = XML::LibXML::XPathContext->new;
$xpc->registerNs('x', 'http://www.w3.org/1999/xhtml');
$xpc->find('/x:html',$node);
Another possibility is to use prefixes declared in the queried document (if known). If the document declares a prefix for the namespace in question (and the context node is in the scope of the declaration), XML::LibXML allows you to use the prefix in the XPath expression, e.g.:
$node->find('/x:html');

**jackthehipster** · Answer 2 · 2014-11-11T16:38:18.310000

I found another solution. Simply using this

say "1:" . $t1->findvalue('//*[local-name()="date"]');

will also find the value and save the hassle of declaring namespaces in an XPathContext. But apart from that, tobyinks answer is the correct one.

Why is this xmlns attribute messing up my xpath query?

There are 2 best solutions below

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