こっちのほうがいいかも: http://www.clear-code.com/blog/2009/7/31.html#.E3.82.AD.E3.83.BC.E3.83.AF.E3.83.BC.E3.83.89.E5.91.A8.E8.BE.BA.E3.81.AE.E6.96.87.E7.AB.A0.E3.81.AE.E8.A1.A8.E7.A4.BA

snippet.rb

#!/usr/bin/env ruby

require "groonga"

db_path = ARGV[0] || File.expand_path("~/.feedcellar/db/feedcellar.db")

Groonga::Database.open(db_path) do |database|
  feeds = Groonga["Feeds"]
  selected_feeds = feeds.select do |feed|
    feed.title =~ /Ruby/
  end

  expression = Groonga::Expression.new
  variable = expression.define_variable(:domain => feeds)
  expression.append_object(variable)
  expression.parse("Ruby", :default_column => :title)
  expression.compile
  snippet = expression.snippet([["<mark>", "</mark>"]], :width => 30)

  selected_feeds.each do |feed|
    p snippet.execute(feed.title)
  end

  snippet.close
end