The amount of white space between paragraphs is generally called "paragraph spacing" or leading. As such it should be handled within the normal text definition of the CSS stylesheet of your ebook. It has nothing to do with RegEx at all.
Your ebook should have a stylesheet included within it; alternately it is at the top of any file which contains the body text of the book. If it's an epub it could be in every chapter. I don't know the exact command, but I believe this is one field that allows em, en, px or % measurements (among others) to be used. As such you might have to play with the values provided, changing the numbers enough to cause an obvious change to the text (add 20 to whatever number is there, see what the outcome is, etc).