Neovim's Expression Register

The expression register is not really a register itself, but it provides a way to evaluate expressions an insert the result into the buffer as if it had been retrieved from a buffer.

This is easiest to understand with a simple example. Starting with an empty buffer we enter insert mode:

then access the expression register:

now we can use the expression register to calculate the value of 2*pi:

The result of the expression (6.28) has been inserted at the cursor location.