How to replace blank lines with a word in Notepad++?

Question

Notepad++

Let's imagine that I have lines:
A

B

C

D

I need to replace the empty lines between them with the same words, so that it turns out like this:
A
X
B
X
C
X
D

In Notepad ++ or in any other text editor. How to do it?

Answer 1

Regular ^$