You'll propably want to use REXML and XPath:

require 'rexml/document'
require 'rexml/xpath'

include REXML

bibliography = Document.new( ARGV[0] )

XPath.each( bibliography, "/biblioentry[pubdate > 1993]") do
|biblioentry|
  # do something with biblioentry here
end

Not entirely sure if that works, as the PC I'm on doesnt have Ruby
installed :(

Scott