Search options are set to: Text + RegEx + Search in Included documents + Search in Excluded documents.
b[/b] or ([ ]$) should locate documents where one or more paragraphs end with a . This finds a couple of documents but misses 99% of them. (Not sure if it’s relevant, but the hits were for non-English text)
b[/b] finds all of them, but \s is not the same as [ ] in RexEx.