I was unable to find [:alnum:] inside core, but I found references and usage ...

I was unable to find [:alnum:] inside core, but I found references and usage of [:alnum:] character class here https://www.hscripts.com/tutorials/regular-expression/character-classes/.... I am not sure whether it's unicode aware. I too was unable to find proper documentation for [:alnum:], in that case I agree we shouldn't use this.