A phrase counts as a palindrome here if it reads the same both ways after ignoring case and every character other than letters and digits.

is_clean_palindrome("No lemon, no melon")
# True
is_clean_palindrome("ab")
# False

You could build a filtered string and compare it with its reverse, but that uses extra memory. Two indexes can do it in place:

• left starts at the beginning and right at the end.
• Move each inward past characters that do not count. str.isalnum tells you which those are.
• Compare the two characters with lower(). A mismatch ends the search.
• Stop when the indexes meet.

Text with nothing to compare, such as "!!" or an empty string, counts as a palindrome.

Write is_clean_palindrome(text) using two pointers, returning True if the letters and digits of text read the same in both directions, ignoring case.

The explanation opens here once every check passes.