String matching, like == or string.compare? Anything more complicated than that and you're not building anything that isn't already available.

The point isn’t to build something new, but to learn CS foundations. Being able to think through this type of problem is absolutely worthwhile.

Why is it worthwhile?

Given finite time, a person could instead learn how to use a new established framework, API, or even programming language.

Because APIs and languages change every few years. Concepts and foundations don't. If I move to a company using a new language, I am good enough to pick it up because the design of my sofware is more about the data structures and their interactions.

The reality is that the majority of companies do hire on the basis of APIs, languages and frameworks.

Sure. But that's not foundational. That's just a job.