I happen to come across the = register. I checked
:h @= and
:h recording where it is mentioned. I am not able to understand how to use it. It could be helpful to know a valid usecase for the same.
So let's begin by quoting the doc
When typing the '=' after " or CTRL-R the cursor moves to the command-line, where you can enter any expression (see |expression|). All normal command-line editing commands are available, including a special history for expressions. When you end the command-line by typing <CR>, Vim computes the result of the expression. If you end it with <Esc>, Vim abandons the expression.
And it is also worth reading
:h usr_41.txt about the expressions.
Now how can you use the expression register?
It is important to understand the the expression register is not meant to be used in a macro, it more common usage is to insert the result of an expression while you are in insert mode.
For example you can do simple math. Let's get this in our buffer:
1 + 2 + 3 + 4 =
If you want to put the result you can do the following:
0 To go the the beginning of the line yf4 To yank the text from 1 to 4 A To start inserting at the end of the line <C-r> Here the magic begins, this allow to insert the content of a register = To start editing the content of the expression register <C-r>" To insert your yanked text in the expression register <CR> To validate the operation
And voila vim computes the content of the expression register (i.e.
1 + 2 + 3 + 4) and inserts the result (i.e.
1 + 2 + 3 + 4 = 10
Now the actual use cases depends on your usage but evaluating some vimscript while you are in insert mode can sometimes be useful.