How can I detect certain Unicode characters in a string in Ruby?

Given a string in Ruby 1.8.7 (without the awesome Oniguruma regular expression engine that supports Unicode properties with \p{}), I would like to be able to determine if the string contains one or more Chinese, Japanese, or Korean characters; i.e.

class String
  def contains_cjk?
    ...
  end
end

>> '日本語'.contains_cjk?
=> true
>> '광고 프로그램'.contains_cjk?
=> true
>> '艾弗森将退出篮坛'.contains_cjk?
=> true
>> 'Watashi ha bakana gaijin desu.'.contains_cjk?
=> false

I suspect that this will boil down to seeing if any of the characters in the string are in the Unihan CJKV Unicode blocks, but I figured it was worth asking if anyone knows of an existing solution in Ruby.