I have an interesting problem where I need to replace a specific instance of a word in a string.

Example String:Fsih my test variable is fsihd or is it gfsih or gfsihd fsih

I may want to replace the third instance of 'fsih' with 'fish', or the fourth, or even the last. How would you do that? I have tried several ways (regexp) but I failed each time.

Thank you in advance

Kristofer