foo.rb

require 'rubygems'
require 'nokogiri'

class MyFilter
  def ends_with set, ends
    set.map { |x| x.to_s }.join.end_with? ends
  end
end

doc = Nokogiri.XML(DATA)
doc.xpath('//foo[ends_with(text(), "ar")]', MyFilter.new).each do |node|
  puts node
end

__END__
<root>
  <foo>bar</foo>
  <foo>baz</foo>
</root>