CSS `foo~:nth-child(2)` gives incorrect XPath

[SimonSapin]

Hi,

I’m the maintainer of cssselect, which does in Python pretty much the same as Nokogiri for CSS selectors: translate them to XPath. It looks like the SimonSapin/cssselect#12 bug also applies to Nokogiri. Namely, the XPath translation of :nth-child() and similar pseudo-classes is wrong when used after the + or ~ combinator. Here is a test case:

require 'nokogiri'
doc = Nokogiri::XML('<root><child1/><child2/><child3/></root>')
puts doc.css(':nth-child(2)').map { |e| e.name }
puts doc.css('child1 ~ :nth-child(2)').map { |e| e.name }

Expected output: child2 child2. Actual output child2 child3.

The problem is in the XPath translation of the later selector: //child1/following-sibling::*[position() = 2 and self::*] gives the element at position 2 when counting from child1, while we want the position among the parent’s children.

I am not sure it is even possible to correctly translate this selector to XPath: the = XPath operator on node-sets compares the text content of elements, not their identity.

The issue is similar for SimonSapin/cssselect#4 and Nokogiri’s #394.

[AurelPaulovic]

you should not use position() which depends on the context position

instead try

//child1/following-sibling::*[(count(preceding-sibling::*) +1)=2]

and similarly for Xn

//child1/following-sibling::*[(count(preceding-sibling::*)+1) mod X = 0]

[SimonSapin]

Thank you @AurelPaulovic , I think that should work.

Now for a selector h2 ~ div:nth-of-type(2) the XPath expression could be //h2/following-sibling::div[count(preceding-sibling::div)=1]. But for the more general case: h2 ~ *:nth-of-type(2), XPath is //h2/following-sibling::*[count(preceding-sibling::*[name(.)=name(…)])=1] is there some expression we could put instead of … to refer to the outer scope? Or maybe a way to bind the outer scope to a variable?

[AurelPaulovic]

Sadly, there is no way how to do that in XPath 1.0. You can't assign any variables and there is no way how to get to the outer context.