solver.rb

class Solver
  def self.solve!(my_array, my_string)
    # my_array.each { |k| my_string.gsub!(k, " #{k} ") }
    # my_string.split.join(' ')

    # Working:
    my_string.gsub(/(#{my_array.sort_by(&:length).reverse.join("|")})\s*/, ' \1').split.join(' ')

    # my_string.gsub(/(#{my_array.join("|")})\s*/, ' \1')
    # my_array.sort_by(&:length).each { |k| my_string.gsub!(/#{k}\s*/, " #{k}") }
  end
end
[
  [
    %w[therapy fun is],
    'therapyisfunfunfunfunis'
  ],
  [
    %w[hello world good bye],
    'worldgoodgoodbye'
  ],
  [
    %w[foo bar baz the therapy],
    "oofrabzabtherapy"
  ]
].map do |my_array, my_string|
  Solver.solve!(my_array, my_string)
end