scrape.rb

%w(open-uri nokogiri).each {|x| require x }
URL = 'http://rubygems.org/gems/rubygems-update/versions'

def get_versions
  versions = []
  doc = Nokogiri::HTML open(URL)
  doc.xpath('//ol/li/a').each do |a|
    versions << a.text
  end
  versions
end

def main
  versions = get_versions
  regexps = versions.map {|v| /^#{Regexp.escape(v)}$/}
  puts Regexp.union(*regexps)
end

main