This indexes the words in a string by length.

index_by_length.rb

# The input.
INPUT = 'The quick brown fox jumped over the lazy dogs. ABCDEF'

# Create a hash that returns an empty Array for missing item accesses.
len_index = Hash.new do |hash, key|
  hash[key] = []
end

# Split the input and store each word in the hash, keyed by the length of the
# word.
INPUT.split(' ').each do |word|
  len_index[word.length] << word
end

# Get the maximum key length.
longest_len = len_index.keys.max
puts "The longest is #{len_index[longest_len].inspect} (#{longest_len})."