Re: [xml-dev] Beware of XPath expressions that produce false positives

[Dimitre Novatchev <dnovatchev@gmail.com>]

Hi Roger,

<quote>

Okay, I added namespace::* to the XPath that David provided, to take into account namespaces. Here is the new XPath: empty((node(),@*,namespace::*))

 

Here are the results I got from running my XSLT program using the new XPath:

 

	Is empty?
<B/>	true
<B></B>	true
<B>&null;</B>	true
<B>  </B>	false
<B><!-- Hello, world --></B>	false
<B><bad/></B>	false
<B>99</B>	false
<B x="10"/>	false
<B xmlns:b="test"/>	false

 

Perfect!

</quote>

I am wondering what combination of parser/xslt-processor you were using...

Because the XPath expression: 

   empty((node(),@*,namespace::*))

should produce  false in all cases.

The reason? every element has at least two namespace nodes -- the xml namespace and the xml-namespace  namespace nodes.

And, indeed, this is exactly what I am getting using Saxon 9.1.

A correct XPath expression is this:

empty( (
             node(),
             @*,
             namespace::*[not(name() = ('xml', 'xmlns'))]
             )
           )

Here I assume that the XML parser will raise an error in case the reserved prefixes "xml" and "xmlns" are rebound to other namespace URIs.

Cheers,

Dimitre

On Sat, Sep 24, 2016 at 2:02 PM, Costello, Roger L. <costello@mitre.org> wrote:

David Carlisle wrote:

Also you are classing attributes as non-empty content
but not namespace nodes so

<B foo=""/> is non empty but
<B xmlns:x="data:,hello"></B> is empty

Ah, good catch David.

Okay, I added namespace::* to the XPath that David provided, to take into account namespaces. Here is the new XPath: empty((node(),@*,namespace::*))

 

Here are the results I got from running my XSLT program using the new XPath:

 

	Is empty?
<B/>	true
<B></B>	true
<B>&null;</B>	true
<B>  </B>	false
<B><!-- Hello, world --></B>	false
<B><bad/></B>	false
<B>99</B>	false
<B x="10"/>	false
<B xmlns:b="test"/>	false

 

Perfect!

 

Those results are from running SAXON in Oxygen XML.

 

John and David pointed out I might get different results with another parser. Specifically, another parser might discard comments and the whitespace in <B>  </B>. Such a parser would produce these results:

 

	Is empty?
<B/>	true
<B></B>	true
<B>&null;</B>	true
<B>  </B>	true
<B><!-- Hello, world --></B>	true
<B><bad/></B>	false
<B>99</B>	false
<B x="10"/>	false
<B xmlns:b="test"/>	false

 

Is there any way to write an XPath expression that always produces the results shown in the first table, regardless of which parser is used?

 

/Roger

 

--

Cheers,

Dimitre Novatchev

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