xml - XPath to find elements that have no other elements referencing them -

given next xml document…

class="lang-xml prettyprint-override"><r> <e id="a" /> <e id="b" /> <e id="c" /> <x ref="#b" /> </r>

…how can write xpath 1.0 look finds <e> elements not have <x> element referencing them? in example, result should #a , #c.

based on this question tried //e[ not(//x[@ref=concat("#",@id)]) ], not omit referenced element:

class="lang-ruby prettyprint-override">

# ruby code using nokogiri puts doc.xpath( '//e[ not(//x[@ref=concat("#",@id)]) ]' ) #=> <e id="a"/> #=> <e id="b"/> #=> <e id="c"/>

is there way utilize attribute in found set farther query values of other attributes in other elements?

from xml

<r> <e id="a" /> <e id="b" /> <e id="c" /> <x ref="#b" /> </r>

this xpath

//e[ not(//x/@ref=concat("#",@id)) ]

will select

<e id="a"/> <e id="c"/>

as requested.

xml xpath nokogiri

Three