# @param {String} s

# @return {Boolean}

def is_palindrome(s)

if s.nil?

return false

end

if s == ''

return true

end

formattedStr = s.downcase

left = 0; right = formattedStr.size - 1

while left < right

unless (formattedStr[left] >= 'a' and formattedStr[left] <= 'z') or (formattedStr[left] >= '0' and formattedStr[left] <= '9')

left += 1

next

end

unless (formattedStr[right] >= 'a' and formattedStr[right] <= 'z') or (formattedStr[right] >= '0' and formattedStr[right] <= '9')

right -= 1

next

end

# print "L_index: #{left}, L_value: #{formattedStr[left]}; R_index: #{right}, R_value: #{formattedStr[right]}.\n"

if formattedStr[left] != formattedStr[right]

return false

end

left += 1

right -= 1

end

true

end

puts is_palindrome 'A man, a plan, a canal: Panama'

puts is_palindrome 'race a car'

puts is_palindrome 'ab'