palindrome.rb

class String
  def palindrome?
    tmp = self.downcase.gsub(/[^[[:alnum:]]]/, '') # you can also use /\W/ but it'll keep the underscores
    tmp == tmp.reverse
  end
end

"madam".palindrome?     # => true
"racecar".palindrome?   # => true
"madam, . ".palindrome? # => true

"02/02/2020".palindrome?     # => true
"11/11/11 11:11".palindrome? # => true

"Madam, I'm Adam".palindrome?                 # => true
"A man, a plan, a canal – Panama".palindrome? # => true

There's no need to use strip at the end, the gsub is already removing the white spaces. And speaking of white spaces, should an empty string be considered a palindrome?

Well done! 👍

The strip method call comes from my previous implementation. Thank you.
Uh.. It's a good question. I think that empty strings should be considered as palindrome. But it's subjective.