Count the number of unique words in a sentence in a case insensitive manner. Words are sequences of letters or digits. Punctuation and separators are ignored. Return 0 for an empty string. Return null for null input.
Examples:
Input: sentence = "Hello, hello world"
Output: 2
Explanation: Unique words are hello and world
Input: sentence = "One two TWO three"
Output: 3
Explanation: Unique words are one, two, three
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