How do I search a field for empty or blank?

[M-a-v]

I'm trying to search a folder of images.. Some of the images have a Embedded comments and some do not.
I just want get a search results for ones that do not have comments.
Is there a way to search that field for "blank' or 'empty'?
I've tried a few of the options, even some Regexp, but I can't seem to get it to work.