Vim search and replace by decrementing back-reference?

Question

Given the following text in Vim:

[2] [3] [4]

I want to perform a search and replace and produce the following:

[1] [2] [3]

I know how to extract out the numbers using back-reference via search and replace:

:%s/\[\(\d\)\]/[\1]/g

But now the question is how do you go about decrementing the value of \1.

Any ideas?

Answer 1

Try

:%s/\[\(\d\+\)\]/\=join(['[', submatch(1) - 1, ']'], '')/g

EDIT: I added a \+ after \d in case you wanted to match more than single digit numbers.

See :help sub-replace-special

Answer 2

Try this:

%s:\d:\r&\r:g

Then

s/\d/\=submatch(0)-1/

And now you need to join the lines.