Detecting when backticks are necessary

Hi, I'm here again with another smarty regexp question :-) I need to detect when it is necessary to add backticks around a smarty variable embedded inside a string. Is there any smarty predefined syntax I could use to do that ? And is it a big overhead if I add backticks around every smarty variable found inside a string ? I have to admit it would be the easiest solution but I don't want to kill performances. Thanks in advance, Katana