String.cyrillic? http://memo.undr.su/2011/11/04/proverka-na-russkie-simvoly-v-stroke/

cyrillic.rb

class TestCyrillic
  class << self    
    def cyrillic?(string)
      string[/\p{Cyrillic}/]
    end
  end
end

class String
  def cyrillic?
    TestCyrillic.cyrillic?(self)
  end
end

test.rb

raise unless "Русские буквы".cyrillic?                            # true
raise unless "Русские буквы и !№%:,.;()_+=-0986654".cyrillic?     # true
raise unless "Русские буквы и English letters".cyrillic?          # true <- was false
raise if "!№%:,.;()_+=-0986654".cyrillic?                     # false <- was true
raise if "Only English letters".cyrillic?                     # false
raise if "English letters and !№%:,.;()_+=-0986654".cyrillic? # false