How to create an expression-rule with RegExp containing alternative expressions in round brackets (result: "Invalid regular expression"?!)

Dear community,

I am trying to create an expression-rule for incoming emails on our sophos central. I'm used to work with regexp since years, but this case drives me crazy: As soon as I try to save a rule with alternatives in round brackets separated by pipe, the rule is rejected with the error "Invalid regular expression".

Easily to reproduce, even with banal simple rule like:

(test123|123test)

I would highly appreciate any ideas.

Thanks

Migo