Check whether a phrase reads the same backwards

A word puzzle site accepts a phrase as a palindrome when its letters and digits read the same in both directions. Spaces, punctuation and case do not count.

Write is_clean_palindrome(s) that returns whether s is a palindrome by that rule.

Input and output:

• s: a string of 0 to 200,000 printable ASCII characters, codes 32 to 126.
• Returns true or false.

Rules:

• Keep only the letters a to z and A to Z and the digits 0 to 9. Ignore every other character, including spaces, punctuation and _.
• Compare letters without regard to case, so A matches a. A digit matches only the same digit.
• Return true when the kept characters read the same forwards and backwards.
• A string with no letters or digits, including the empty string, returns true.