Strict and non-strict palindrome. Default is not strict.

Palindrome.rb

#!/usr/bin/env ruby
class String
  def palindrome?(strict = false)
    return true if length.zero? || length == 1
    i = -1
    j = length
    length.times do
      loop do
        i += 1
        break if strict || self[i] =~ /[A-z0-9]/ 
      end

      loop do
        j -= 1
        break if strict || self[j] =~ /[A-z0-9]/
      end
      
      return false unless self[i].casecmp(self[j]).zero?
      return true if i >= j - 2
    end
    return false
  end
end

# palindromic poem by Demetri Martin
poem = <<~POEM
  Dammit I'm mad.
  Evil is a deed as I live.
  God, am I reviled? I rise, my bed on a sun, I melt.
  To be not one man emanating is sad. I piss.
  Alas, it is so late. Who stops to help?
  Man, it is hot. I'm in it. I tell.
  I am not a devil. I level "Mad Dog".
  Ah, say burning is, as a deified gulp,
  In my halo of a mired rum tin.
  I erase many men. Oh, to be man, a sin.
  Is evil in a clam? In a trap?
  No. It is open. On it I was stuck.
  Rats peed on hope. Elsewhere dips a web.
  Be still if I fill its ebb.
  Ew, a spider… eh?
  We sleep. Oh no!
  Deep, stark cuts saw it in one position.
  Part animal, can I live? Sin is a name.
  Both, one… my names are in it.
  Murder? I'm a fool.
  A hymn I plug, deified as a sign in ruby ash,
  A Goddam level I lived at.
  On mail let it in. I'm it.
  Oh, sit in ample hot spots. Oh wet!
  A loss it is alas (sip). I'd assign it a name.
  Name not one bottle minus an ode by me:
  "Sir, I deliver. I'm a dog"
  Evil is a deed as I live.
  Dammit I'm mad.
POEM

# `if poem.palindrome? true` for strict palindrome check.
check = if poem.palindrome? # non-strict palindrome check.
  "Yay! A palindrome."
else
  "Sadly, not a palindrome."
end

puts check