Paiza レベルアップ問題集: 「日付のフォーマット Ruby編」 - https://paiza.jp/works/mondai/dateset/ruby/date_format

date_format.rb

class DateFormat
  DATE_ORDER = %r{[0-9]{4}/[0-9]{2}/[0-9]{2}}.freeze
  MONTH = [*(1..12).map { |n| n.to_s.rjust(2, '0') }].freeze
  DAY   = [*(1..31).map { |n| n.to_s.rjust(2, '0') }].freeze

  class << self
    def validate(date_string)
      return 'No' unless date_string.match?(DATE_ORDER)

      _year, month, day = date_string.split('/')

      return 'No' unless MONTH.include?(month)
      return 'No' unless DAY.include?(day)

      'Yes'
    end
  end
end

puts DateFormat.validate(gets.chomp)