What does <c-r>= means in Vim

Question

I came across a number of this syntax usages which I don't understand:

The first is in :help mapping:

:map <F2> a<C-R>=strftime("%c")<CR><Esc> 

This sequence really does insert the value of strftime into buffer though I don't understand how. Changing onto something different breaks it.

Another one is at wiki page which describes how to make omnicompletion popup menu work well:

inoremap <silent> <Esc> <C-r>=pumvisible() ? "\<C-y>" : "\<Esc>"<CR> 

The same thing here.

Can anybody explain how this "<C-r>=" thing works?...

Answer 1

<C-r>=, or Ctrl+R= is used to insert the result of an expression at the cursor.

I use it a lot when editing CSS to insert values:

width: <C-r>=147-33<CR>px; width: 114px; 

EDIT

<C-r>, without =, allows you to insert the content of any register at the cursor while staying in insert mode: <C-r>+, for example, inserts the content of my system clipboard. see :help i_ctrl_r.

= is the "expression register". See :help "=.

ENDEDIT