Dr C's tip is a great start: I prefer to use </p>
\s+<p to allow for code indention variations
There will (always) be exceptions
Mr.</p>
<p>Jones
I have a series of these REGEX patterns I use
The one given is my usual first pass (use ALL with care. There are EXCEPTIONS

where the lines should NOT be joined)
IMHO Always do a bunch of Find, Replace manually before letting the REGEX loose (if at all) to see if there are Gotchas in your pattern