In insert and command mode,
<Ctrl-r> followed by a sequence inserts an object.
Insert mode usage mainly deals with manipulating registers. However, in command mode,
<Ctrl-r><Ctrl-*> inserts special objects for certain
F: Current filename under cursor
P: Current filename with expanded path under cursor
W: Word under cursor
A: WORD under cursor
L: Line under cursor
Very useful with
:Ag, or when doing find/replace.