def is_palindrome(s)
  str = s.downcase.chars.select { |char| /[a-zA-Z0-9]/.match?(char) }.join
  idx_start = 0
  idx_end = str.length - 1
  while idx_start < idx_end
    return false if str[idx_start] != str[idx_end]

    idx_start += 1
    idx_end -= 1
  end
  true
end

def is_palindrome(s)
  idx_start = 0
  idx_end = s.length - 1
  while idx_start < idx_end
    if !/[a-zA-Z0-9]/.match?(s[idx_start])
      idx_start += 1
    elsif !/[a-zA-Z0-9]/.match?(s[idx_end])
      idx_end -= 1
    else
      return false if s[idx_start].downcase != s[idx_end].downcase

      idx_start += 1
      idx_end -= 1
    end
  end
  true
end

def is_palindrome(s)
  str = s.downcase.chars.select { |char| /[a-zA-Z0-9]/.match?(char) }
  str == str.reverse
end