Compliance-Grade Palindrome Sanitizer

Problem Narrative

You’re building a text-ingestion service for a fintech company that processes customer support messages for dispute resolution and regulatory audits. The pipeline handles tens of millions of messages/day, and a downstream classifier relies on a fast, deterministic check to flag messages that read the same forward and backward after normalization (a common signal in spam and obfuscation).

Because these messages may contain punctuation, whitespace, and mixed casing, your palindrome check must be normalization-aware and efficient.

Task

Implement a function:

• Input: s: str
• Output: bool

Return True if s is a palindrome after applying the following normalization rules:

1. Consider only ASCII alphanumeric characters: [A-Z], [a-z], [0-9].
2. Compare letters case-insensitively.
3. All other characters (spaces, punctuation, symbols) are ignored.

A palindrome reads the same forward and backward after normalization.

Notes / Clarifications

• An empty string (or a string with no alphanumeric characters) should return True.
• You should aim for a solution that runs in a single pass over the string with O(1) extra space.
• Do not build a fully normalized copy of the string unless you choose an alternative approach.

Constraints

• 0 <= len(s) <= 2 * 10^5
• s contains printable ASCII characters