words = File.readlines('words_alpha.txt').map do |word|
  word.strip.upcase
end.select do |word|
  word.size == 5
end

neighbours = Hash.new { |h, k| h[k] = [] }
words.each do |word|
  5.times do |i|
    neighbours[word[...i] + "-" + word[i++1..]] << word
  end
end

neighbours.values.select{|v| v.size > 1}