Help with Search & Replace function and fi/fl ligatures
 

Converting from epub to azw3, I have calibre set up to automatically replace fi and fl with their proper unicode ligature.

But I found some words if played with will mess up the code of the book, for example <figurecaption> is used for image captions, and the fi in figurecaption should remain the characters f and i.

How do I tell calibre to replace the fi character with the ligature but only if it's not in the word "figure"?