With "?=" you could implement kind of "and" logic, which will make the problem easier, since numbers, which and divisible by 6 are divisible by 2 and by 3.

For the quantifiers I agree with you, they would be handy. On the other hand: this is probably nothing, what is generated "automatically", this would be an outcome of some kind of optimization, but - if I understood it correctly - the goal is to generate the regex on the fly.
(tivia: if allowing quantifiers, all digits needs to be allowed as well.)