Ruby implementation of a pyramid with a custom height

pyramid.rb

#
# Input: Pyramid's height
# Output: Pyramid with given height and width = height * 2 - 1
#

MAX_HEIGHT = 20
prompt = 'Enter pyramid\'s height: '
puts prompt

while height = gets.chomp
  case height
  when /^\d+$/
    height = height.to_i
    if height <= MAX_HEIGHT
      height.times do |row|
        level = ''.center 2 * (height - row) - 1, '*'
        puts level.center level.size + 2 * row
      end
      break
    else
      puts "Too big value! #{prompt}"
    end
  else
    puts "Not a natural number! #{prompt}"
  end
end