Valid palindrome

Check whether a string reads the same both ways, ignoring punctuation and case.

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def is_palindrome(s):
    l, r = 0, len(s) - 1
    while l < r:
        if not s[l].isalnum(): l += 1
        elif not s[r].isalnum(): r -= 1
        elif s[l].lower() != s[r].lower(): return False
        else: l += 1; r -= 1
    return True