common.rb

class String
  def substrings
    ((0...length).to_a.combination(2).map do |from,to|
      # Extract substrings
      self[from, to]
    end + self.chars).map do |s|
      # Trim off any unwanted characters
      s.sub(/\A\W+/, '').sub(/\W+\z/, '')
    end.sort.uniq.reject do |s|
      # Remove empty strings
      s.length == 0
    end
  end

  def phrase_in_common(other)
    [ self, other ].map(&:downcase).map(&:substrings).reduce(&:&).max_by(&:length)
  end
end