Vim decremental replace not working

Question

I want to replace the lines :

https://www.dailymotion.com/video/whatever_1 # The Serie - S2 E4 - Grin and Bear It
https://www.dailymotion.com/video/whatever_2 # The Serie - S2 E3 - Betty's Wait Problem
https://www.dailymotion.com/video/whatever_3 # The Serie - S1 E21 - Secretaries Day
...

by these lines :

https://www.dailymotion.com/video/whatever_1 # The Serie - S2 E4 - whatever_79
https://www.dailymotion.com/video/whatever_2 # The Serie - S2 E3 - whatever_78
https://www.dailymotion.com/video/whatever_3 # The Serie - S1 E21 - whatever_77
...

So I tried different combinations of the following vim command :

:let i=79 | g/^http/s/\(S[0-9]\+ E[0-9]\+\).*$/\1 - whatever_\=i/ | let i=i-1

but none has worked and I got this instead :

https://www.dailymotion.com/video/whatever_1 # The Serie - S2 E4 - whatever_=i
https://www.dailymotion.com/video/whatever_2 # The Serie - S2 E3 - whatever_=i
https://www.dailymotion.com/video/whatever_3 # The Serie - S1 E21 - whatever_=i

Can you help me ?

Answer 1

If you want to use \=, it needs to be at the start of the replacement, and the whole replacement expression becomes an expression.

You can use the submatch() function to access your backreferences, since \1 will not work from a \= replacement expression.

After you do that, all you need is " - whatever_".i to produce the replacement using a string concatenation.

Putting it all together:

:let i=79 | g/^http/s/\(S[0-9]\+ E[0-9]\+\).*$/\=submatch(1)." - whatever_".i/ | let i=i-1

It's easier to do that if you don't need the backreference. In this particular case, you can easily avoid the backreference by using \zs, which marks the start of the match, the part that actually gets replaced:

:let i=79 | g/^http/s/S[0-9]\+ E[0-9]\+\zs.*$/\=" - whatever_".i/ | let i=i-1