A method to obtain a list of matching indexes for a substring (or regex).

list_of_matching_indexes.rb

class String
  def list_of_matching_indexes_for substring
    array = []
    self.scan substring do |letter| 
      array << $~.offset(0).first
    end

    array
  end
end

def self.my_test
  text      = 'Potato or tomato or pistachio'
  substring = 'or'

  text.list_of_matching_indexes_for substring
end

my_test == [7, 17]