word_finder.rb

str1 = 'abc def fegh '
str2 = 'feg qwe eerwr feg'

class WordDivider
  MAX_WORD_SIZE = 15
  MIN_WORD_SIZE = 1

  attr_accessor :text, :substrings

  def initialize(string)
    @text = string
    @substrings = substringify!
  end

  def intersect(string)
    substrings.select { |x| string.include?(x) }.sort_by(&:length).reverse
  end

  private

  def substringify!
    ((MIN_WORD_SIZE - 1)..MAX_WORD_SIZE).each_with_object([]) do |i, arr|
      (0..(text.length - i)).each do |j|
        string = text[j..j + i]
        next if WordValidator.new.validate(string) || arr.include?(string)
        arr << string unless arr.include?(string)
      end
    end
  end
end

class WordValidator
  def validate(str)
    str.empty? ||(str =~ /\W/)
  end
end

str_array = WordDivider.new(str1).intersect(str2)