hpricot vs nokogiri xpaths on intermediate nodes

gistfile1.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'nokogiri'
require 'hpricot'

puts "\n\n"

$global_binding = binding()
$global_stmt_count = 0
def print_eval(str)
  $global_stmt_count +=  1
  val = eval(str, $global_binding)
  puts "#{$global_stmt_count} : #{str}"
  puts "\n"
  puts "  ==> #{val}\n\n\n"
end

xml = <<EOS
<root>
  <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>
    
    <child>
    </child>
  </parent>
  
  <parent>
  </parent>
</root>
EOS

print_eval 'hp_parent = Hpricot::XML(xml).at("//parent")'
print_eval 'nk_parent = Nokogiri::XML(xml).at("//parent")'

print_eval "hp_parent.at('/child')"
print_eval "nk_parent.at('/child')"

print_eval "hp_parent.at('/root/parent')"
print_eval "nk_parent.at('/root/parent')"

print_eval "hp_parent.at('child')"
print_eval "nk_parent.at('child')"
print_eval "nk_parent.at('child/grandchild')"
print_eval "nk_parent.at_xpath('child/grandchild')"

print_eval 'hp_parent.at("./child")'
print_eval 'nk_parent.at("./child")'

print_eval 'hp_parent.at("./child/grandchild")'
print_eval 'nk_parent.at("./child/grandchild")'

print_eval 'hp_parent.at("./child/wrong")'
print_eval 'nk_parent.at("./child/wrong")'

1 : hp_parent = Hpricot::XML(xml).at("//parent")

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


2 : nk_parent = Nokogiri::XML(xml).at("//parent")

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


3 : hp_parent.at('/child')

  ==> <child>
      <grandchild>
      </grandchild>
    </child>


4 : nk_parent.at('/child')

  ==>


5 : hp_parent.at('/root/parent')

  ==>


6 : nk_parent.at('/root/parent')

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


7 : hp_parent.at('child')

  ==> <child>
      <grandchild>
      </grandchild>
    </child>


8 : nk_parent.at('child')

  ==> <child>
      <grandchild>
      </grandchild>
    </child>


9 : nk_parent.at('child/grandchild')

  ==> <grandchild>
      </grandchild>


10 : nk_parent.at_xpath('child/grandchild')

  ==> <grandchild>
      </grandchild>


11 : hp_parent.at("./child")

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


12 : nk_parent.at("./child")

  ==> <child>
      <grandchild>
      </grandchild>
    </child>


13 : hp_parent.at("./child/grandchild")

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


14 : nk_parent.at("./child/grandchild")

  ==> <grandchild>
      </grandchild>


15 : hp_parent.at("./child/wrong")

  ==> <parent>
    <child>
      <grandchild>
      </grandchild>
    </child>

    <child>
    </child>
  </parent>


16 : nk_parent.at("./child/wrong")

  ==>