you can not come back every 2 years.

Given you 'RI' (PR as you may call) means you've choosen this country as you long term residential country. If you leave this country up to 2 years for th first time. You may lucky to get in with a warning. If you do it again the very next time then you will be required a visa before entry.
You got to be very careful and better contact Homw office to confirm. There isn't a stamp on your passport when you leave but they have scan your passport to record into data system.