String#emphasize puts <em>'s in strings

String#emphasize

class String
  def emphasize substrings
    cleaned = [substrings].flatten.compact.uniq.map(&:strip).reject(&:empty?)
    return self.clone if cleaned.empty?
    returning to_s do |em|
      cleaned.inject em do |s, sub|
        s.gsub!(%r/(#{sub})/i, '<em>\1</em>') || s
      end
    end
  end
end