m.rb

def rule_one string
  return [string + "U"] if string[-1] == 'U'
  return false
end

def rule_two string
  return ["M" + (string[1..-1] * 2)]
end

def rule_three string
  n = 2
  results = []

  while n <= string.length
    new_string = string.clone

    if new_string[(n-2)..n] =~ /III/
      new_string[(n-2)..n] = 'U'

      results.push new_string
    end

    n += 1
  end

  return results
end


def rule_four string
  n = 1
  results = []

  while n <= string.length
    new_string = string.clone

    if new_string[(n-1)..n] =~ /UU/
      new_string[(n-1)..n] = ''

      results.push new_string
    end

    n += 1
  end

  return results
end