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The RegEx there will replace any character not in the set with nothing, effectively removing all but the characters of interest. Once that is complete, use the length( function to count the characters.
There are multiple places(here's one) where you can find ASCII charts to help you map the decimal range (40-45) to the corresponding hexadecimal range (\x28-\x2d), that @Claje (and now @MarqueeCrew) used in their regex formula