test.rb

# irb(main):001:0> load "test.rb"
# => true
# irb(main):002:0> is_kaibun?("あいうえお")
# => false
# irb(main):003:0> is_kaibun?("あいういあ")
# => true
# irb(main):004:0> is_kaibun?("わたしまけましたわ")
# => true
# irb(main):005:0> is_kaibun?("abc")
# => false
# irb(main):006:0> is_kaibun?("abcba")
# => true
# irb(main):007:0> is_kaibun?("abba")
# => true

def is_kaibun?(str)
  # 奇数長の文字列だった場合、真ん中の文字を削除
  if (str.length % 2) == 1
    str = str[0, str.length/2] + str[(str.length/2)+1, str.length]
  end
  str[0, str.length/2] === str[str.length/2, str.length].reverse
end