is_palindrome method ruby

class String
  def is_palindrome?
    str = self.gsub(/[^a-zA-Z]/i, '').downcase
    str_size = str.size

    raise "Incorrect string" if str_size < 2

    half_string_1 = str.slice(0...str_size/2)
    half_string_2 = str.slice(-(str_size/2)..-1)

    if half_string_2 == half_string_1
      puts "It's a palindrome"
    else
      puts "It's not a palindrome"
    end
  end
end

I could write better lol. Even if using the reverse method is prohibited
t.2020