In the hub, a Custom Transformer can be downloaded to count the number of characters in a string: https://hub.safe.com/publishers/gisinnovationsb/transformers/specificcharactercounter This involves creating a list, so it is rather slow.
@takashi has some excellent ideas in: https://community.safe.com/s/question/0D54Q000080hYeESAU/count-number-of-unique-characters-into-a-string
Also some really good ideas in: https://community.safe.com/s/question/0D54Q000080hIUkSAM/count-the-number-of-characters-and-numbers-in-a-given-string
It might be useful though, to have a dedicated Transformer 'CharacterCounter'. I've looked for it several times, before I started implementing a solution with the ExpressionEvaluator. Especially when you can make it faster than the current solutions.