Compare attribute values using Xpath

Question

Given the following document structure, how could i check if two attribute values match using Xpath?

<document lang="en">
<element lang="en"></element>
<element lang="sv"></element>
<element lang="fr"></element>
</document>

What i am looking for is something like:

//document[@lang="[//element[@lang]"]

Answer 1

This will return all <document> nodes having lang attribute value match any child node <element>'s lang attribute :

//document[@lang = element/@lang]

Answer 2

Specifically to your example, you can use:

//document[@lang=child::element/@lang]

If you are just checking whether a match exists, you can wrap it in a boolean:

boolean(//document[@lang=child::element/@lang])

If you want to select the matched element, you can check by ancestor:

//element[@lang=ancestor::document[1]/@lang]

If you want to match any nodes that have matching attributes elsewhere, you can do something like this:

//node()[@lang=following::node()/@lang]

That should match the first node that has a match elsewhere in the document.