Useful tip - replacing a null field

I've a report I am currently writing where it sometimes returns an empty field - this is a list of all data backed up in a particular pool and sometimes the pool field returns blank which doesn't seem right but that is a different problem!  What I want to do is replace the blank field for pool with the text "unknown pool".

I can use the NULL to zero operator and this will replace these fields with 0 so I know these fields are null.  However I want to replace the null field with a string so I use the Search and Replace operator and for a null field I enter in the Search String:

^$

Then in the replace field the text I want to replace this with:

unknown pool

One other point on this operator, I wasted a lot of time because the order the parameters are given are not how you would logically expect them to be arranged!