Problem

In a Meta developer tooling surface such as Phabricator, text normalization often matters before validation. Write a function that returns True if a string is a palindrome after ignoring all non-alphanumeric characters and treating uppercase and lowercase letters as equal; otherwise return False.

Formal Specification

• Input: A string s
• Output: A boolean
• A character should be considered only if it is alphanumeric (a-z, A-Z, 0-9).
• Comparison must be case-insensitive.

Examples

Example 1

• Input: s = "A man, a plan, a canal: Panama"
• Output: True
• Explanation: After removing special characters and lowercasing, the string becomes "amanaplanacanalpanama", which reads the same forward and backward.

Example 2

• Input: s = "race a car"
• Output: False
• Explanation: The normalized string is "raceacar", which is not a palindrome.

Constraints

• 0 <= len(s) <= 2 * 10^5
• s may contain letters, digits, spaces, punctuation, and symbols
• Aim for a linear-time solution
• Prefer constant extra space beyond the input string