Nobody explained the error. Allow me.

Part of your RE was a lookbehind to see that you weren't preceeded by \".*\". Finding that might take searching back 5 characters or 50. Getting that logic right is hard, and it has never been implemented in Perl. Therefore you can only do a lookbehind when Perl can figure out exactly how many characters back it has to go.

(Just to annoy people, Java has no such limit.)