Vim - Adding empty line between lines

Vim's :g command is designed for exactly this sort of task; running a single action on every line which matches a particular pattern. Here's my answer:

:g/.\n\n\@!/norm o

The pattern I use is /.\n\n\@!/. Breaking that down into its component pieces:

• . Matches any character in the line. (used to immediately discard any existing empty lines from consideration)
• \n Matches a single \n at the end of the character above
• \n\@! Fails the match if there's another \n immediately after the earlier \n.

(Check :h E59 for more information on \@! and similar match specifiers in regular expressions -- there are a couple others, too!)

So the :g command's regex has now selected every non-empty line which is terminated by a single newline, and which isn't followed by an empty line.

After the pattern in a :g statement comes the command to run on matching lines. In this case, I've told it to execute a normal mode command (abbreviated to norm), and the command to run is simply o, which inserts an empty line below the current line.

So taken together, the command finds every line which doesn't have an empty line beneath it, and adds one. And that's all there is to it! You might like to check the vim wiki's Power of G article for more fancy things you can do with :g (and it's negative sister, :v) -- it's one of those amazingly useful commands which you soon grow to rely on, and then miss terribly in editors which don't have it.

When I tested @NeilForester's global search and replace answer, I found that it missed every second intended replacement in consecutive non-blank lines if the lines only had one character in each. This appears to be due to the pattern starting to match on each occasion after the last character that it matched the previous occasion.

Using lookbehind and lookahead solves this problem and makes the regex a bit shorter too:

:%s/\n\@<!\n\n\@!/\r\r/g

Another way:

%s/\(.\)\n\(.\)/\1\r\r\2/

Which means,

\(.\) match and capture a non-newline character,
\n    match a newline
\(.\) match and capture a non-newline character

replace with: first capture, double newlines, and the second capture.

For example, line1\nLine2 results in \1 = 1 and \2 = L.