Remove single line breaks but keep empty lines with Vim

Question

Is it possible to remove single line breaks but to keep empty lines (double line breaks I guess?) in Vim? I'm using Vim's HardPencil plugin to do some word processing, and I like the hard line breaks because it makes it easier to bounce around the file. However, I need to share this information in LibreOffice Writer or Word, and hard line breaks won't work well if others edit the file. To clarify, this is what I have:

Line1.
Line2.
Line3.

Line4.
Line5.

And this is what I would like:

Line1. Line2. Line3.

Line4. Line5.

Essentially hard line breaks should be replaced with a space, and double line breaks should be preserved.

I've tried variations of :%s/\n but can't get the double line breaks to be preserved. I've found similar questions here and here, but these are not Vim specific.

Answer 1

You can replace line+line break with just line, and use negative look ahead \(\n\)\@! to make sure it's not followed by an empty line:

:%s/\(.\+\)\n\(\n\)\@!/\1 /

@Peter_Rincker's cleaner solution:

:%s/.\+\zs\n\ze./ /