Question

I was stumbling over a behaviour in Vim’s substitute-command that I can’t
really follow:

Given the following line of code that contains the && sequence I wanted to
substitute the && with && and a newline:

return a && b

In my first try I simply used s/&& /&&^M/g (^M was inserted via Ctrl-V
<Enter>).
This results in the following code:

return a && && 
b

How exactly is the substitution performed to insert the second &&<space>?
I expected the first (and only match) to be &&<space> and this complete match to be
substituted by &&^M?

The question is not about how to perform the correct substitution – I ended up
using the \zs token to reset the start of the match and only insert the
newline.
I just want to understand why exactly the duplication is introduced.

Answer 1

The problem you’re hitting is that the substitute command treats & as a special token in the replacement to mean the matched text. You need to escape it.

:s/&& /\&\&^M/g