I just tried it with both:
Code:
<(p[^>]*|span[^>]*)>(\s|\u00a0|</?\s?br\s?/?>)*</?(p|span)>
then
<(p[^>]*|span[^>]*)>(\s|</?\s?br\s?/?>)*</?(p|span)>
The second was to remove completely. neither one worked. The empty tags still remain in the epub after conversion.