Too much unnecessary code. The Where method already returns an implementation of IEnumerable, which can be returned as a result.
Also, you ignored the requirement "non-negative integers".
In addition, your method returns any type except string, which corresponds to the conditions of the task, but it would be more reliable to return only values of an integer type.

daamn , thank you for this one !

Maybe I'm crazy, but I love Regex, it's so much fun tbh

I don't think anybody wants to willingly torture themselves

Are you kidding? Where is Regex?

Regex ?

While this solution is good to get the job done and might be more efficient, I think the point of this Kata was to explicitly use Regex.

Absolutely.
The spirit of the exercice is tu use reg expressions and it's also more direct, even though Regex is somehow a bit more complex.

Maybe we should add this to the edgecases.