Don't forget to add a space! Or you'll be spell checking for days because you stuck two words together at each join.
It's easy to switch all occurrences of multiple spaces to one space, though, if you happen to double up. So first...
Find:
Replace:
which will concatenate your lines, then...
Find:
Replace:
The second regex above should preserve punctuation that has two spaces (or more) following it. It also won't find the extra space that follows should a hyphen, colon, semi-colon, etc., or some other non-lower-case-letter, somehow be at the end of a line that's joined.
Try it on a copy first.
m a r