I believe he's saying you'd want:

  (?=((?!exp).)*a.*)

I'm a little dubious that full equivalence is maintained (particularly w.r.t. to e.g. #match), but my brain isn't up to handling convoluted REs this early.

Edit: Another hangup I have is how would you do something like:

  ((?~E1)|E2)E3

Obviously you can distribute by duplicating E3, but then it's pretty clear what the absent operator buys you.