Re ensuring the regex is syntactically valid - ok, no big deal.
Counting ( ) would have been my idea of a “solution” for top-level | as well to take this one step further.
BTW CKM is not dying in that case, it would just show you a (false positive) validation error you can however ignore if necessary.
I - of course - agree that what you are suggesting for a slot regex is valid, but at the same time I am not sure it is a good idea in practice. Ensuring that the main use cases are covered and use an agreed syntax where possible (as Ian suggests) that can be used by tooling to provide better explanations etc. seems far more powerful.