If that is what you want, then that is what you want. But the cure is worse than the disease. 
/^(?>(?:a(?{
         $count--
     })|b(?{
         $count++
     }))+)(??{
         $count==1 ? "" : "a"
     })\z/

[download]
Don't forget to have a local our $count; in front of that. And don't forget to buy a bullet-proof vest in case the maintenance programmer ever tracks you down.

In reply to Re: Integrating match counts into regex matching by tilly
in thread Integrating match counts into regex matching by pat_mc

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