See my last edit, I have now added the clarification, that () is reserved only to define tokens (because otherwise, we don't know what is token or not in the interpretation, so we have to reinforce this with an error right?).

Unless you can come up with an example where you want to have () also for either/or, without making this illegal.
E.g., do you want to keep the possibility of doing instead of '(ASL|T1w)' '(ASL(01|02)|T1w)' ? Because '(ASL|T1w)(01|02)' would definitely not work (the 01 or 02 would become a token). In this situation, the user should just use '(ASL|T1w).*', because it otherwise is unclear, 01|02 could also be used for series? So "tokens within tokens" should be illegal, and () can only define tokens, right?