$_ = '(($a $a hi) $a ($a) $a) $a ($a) $a ';
my $re;
($re=$_) =~ s/((\()|(\))|.)/${['(','']}[!$2]\Q$1\E${[')','']}[!$3]/gs;
$re = join'|',map{quotemeta}eval{/$re/};
die $@ if $@ =~ /unmatched/;
s/($re)|\$a/$1?$1:'$b'/eg;