I dont think you can. I believe CAPS-LOCK is probably translated by the OS before vim ever sees it. So you'd need to do a hack at the OS level, like the registry hacks you've already seen.

EDIT: autohotkey looks like it could be used to bridge the vim-OS gap. This way a thirdparty app is doing the hacks at the OS level, and you're just hooking that app.

Capslock (and Control, and Shift etc.) is a modifier key, which means that it's used with another normal key to modify the meaning of that key. AFAIK the OS does not pass the modifier keys to the application unless a normal key has also been pressed, e.g. pressing CTRL will not be seen by the application, but CTRL-C will be.

Under windows? Use AutoHotkey. It's not a vim mapping, but as the others have stated you can't map it. I use AHK to map my CAPSLOCK to CTRL.

Linux? With X, use xmodmap to alter the key mapping, e.g.

xmodmap -e 'clear Lock' -e 'keycode 0x42 = Escape'

Will map Esc to the CapsLock key. Google for more examples.

Since there is a solution for Linux and Windows(Autohotkey), I´d like to suggest to use pckeyboardhack for Mac to remap CapsLock everywhere.

If your intention is just to avoid working outside of Vim, you can put these lines in your .vimrc:

au VimEnter * !xmodmap -e 'clear Lock' -e 'keycode 0x42 = Escape'
au VimLeave * !xmodmap -e 'clear Lock' -e 'keycode 0x42 = Caps_Lock'

The first line maps escape to the caps lock key when you enter Vim, and the second line returns normal functionality to caps lock when you quit.

This requires Linux with the xorg-xmodmap package installed.

For Mac OS, you can remap the 'caps lock' key system wide in 'system preferences'.

Follow this path:

system preferences > keyboard > modifier keys

Then click the drop down box next to 'caps lock' and choose '^ Control'.

In Linux systems this can be done with xmodmap.

Save this in a text file in the home folder

! Swap caps lock and escape
remove Lock = Caps_Lock
keysym Escape = Caps_Lock
keysym Caps_Lock = Escape
add Lock = Caps_Lock

Save this file with a name like .capstoescswitc

Then execute this file via the terminal.

xmodmap ~/.capstoescswitc

If want to reveres it simply switch the key variables in the script file.

For more info refer this page

Solution that doesn't break Caps Lock outside of vim

Windows

1. Install autohotkey.
2. Run autohotkey script:

;caps_to_esc.ahk
#IfWinActive, ahk_class Vim ; vim window class
Capslock::Esc
#IfWinActive

Ubuntu

Run this command:

wget -O - https://raw.githubusercontent.com/grabantot/scripts/master/install/install_caps_to_esc.sh | bash

Or perform these actions manually:

1. sudo apt-get install xdotool xbindkeys. We will also use xprop and xset (should be installed by default).
2. Create a ~/caps_to_esc.sh script:

debug_file=/dev/shm/caps_to_esc.debug
debug_msg () {
echo $(date +%s%3N) "$@" >> $debug_file
}


caps_off () {
is_caps_on="false"
xset q | grep "Caps Lock:\s*on" && is_caps_on="true"
debug_msg "is_caps_on ""$is_caps_on"


[ "$is_caps_on" == "false" ] && return 3
debug_msg "Sending Caps Lock"
debug_msg "ignore_next"
xdotool key Caps_Lock
}


should_ignore="false"
tail -n 1 $debug_file | grep "ignore_next" && should_ignore="true"


if [ "$should_ignore" == "true" ]; then
debug_msg "ignored"
exit 1
fi


echo -n "" > $debug_file


# get wm_class by 'xprop | grep WM_CLASS'
declare -a wm_classes=( \
'WM_CLASS(STRING) = "gnome-terminal-server", "Gnome-terminal"' \
'WM_CLASS(STRING) = "gvim", "Gvim"' \
'WM_CLASS(STRING) = "code", "Code"' \
'WM_CLASS(STRING) = "google-chrome", "Google-chrome"' \
)


active_window_id=$(xdotool getactivewindow)
active_window_wm_class=$(xprop -id $active_window_id WM_CLASS)
debug_msg "active_wm_class   ""$active_window_wm_class"


detected_wm_class=""
for wm_class in "${wm_classes[@]}"; do
# debug_msg "$wm_class"
if [ "$active_window_wm_class" == "$wm_class" ]; then
detected_wm_class="$wm_class"
debug_msg "detected_wm_class ""$detected_wm_class"
fi
done


[ "$detected_wm_class" == "" ] && exit 2
xdotool keyup "Caps_Lock" # !!! very important
caps_off
debug_msg "Sending Escape"
xdotool key "Escape"
debug_msg "sent"

3. Add new bindnig to ~/.xbindkeysrc:

"bash $HOME/caps_to_esc.sh"
Caps_Lock

4. killall xbindkeys && xbindkeys

How it works:

1. xbindkeys will detetect when Caps_Lock is pressed and call caps_to_esc.sh script
2. in the script detect active window wm_class by xprop
3. check if wm_class is of interest for us (gnome-terminal, vscode, gvim, chrome), exit if it is not
4. send Escape key via xdotool
5. check if Caps Lock is on via xset and if it is then send Caps_Lock key via xdotool
6. xbindkeys will detect the Caps_Lock sent by us but we ignore it

I guess one of the reasons for doing this is to create a soft capslock, like others have mentioned, possibly to avoid keeping capslock on while in normal mode. I've used the vimcaps plugin to turn off capslock when leaving insert mode, seems to work okay.

On mac, it is also possible to use Karabiner (https://pqrs.org/osx/karabiner/)

$ brew cask install karabiner-elements

Once installed, you can map capslock key to esc key in the simple modifications tab. Caveat is this is system wide, meaning that you lose capslock key everywhere. IMO who needs capslock.

A working example for a mere Mortal WINDOWS user like me as of 2020 is using AutoHotkey

Currently using this 1 line script:

Capslock::Esc

Run it and your problem is solved. Hope it helps. :)

@rsoren's answer works. But the problem with that is if multiple buffers are opened, exiting from one, reverts the mapping for all of the others too. Replacing VimEnter and VimLeave with BufEnter and BufLeave, did the trick for me.

au BufEnter * silent! !xmodmap -e 'clear Lock' -e 'keycode 0x42 = Escape'
au BufLeave * silent! !xmodmap -e 'clear Lock' -e 'keycode 0x42 = Caps_Lock'

Remap CAPSLOCK to ESC and CTRL on Windows

If you want to remap CAPSLOCK to both

1. to ESC (when pressed alone)
2. to CTRL (when pressed with other keys) you can use this little piece of open source software by ililim.

You do not need Admin privileges for this and ESC and CTRL are still working as expected.

I use it to enjoy convenient CTRL + anything presses without hurting my pinky and to toggle modes in Vim that I use via ssh in MobaXterm. For installation just follow this description.

Remap CAPSLOCK to ESC and CTRL on Linux

To do the same on Linux, you can use XCAPE. To install it on Ubuntu use:

sudo apt install xcape

For the actual mapping do:

setxkbmap -option ctrl:nocaps
xcape -e 'Control_L=Escape'

The 1st command is to map CAPSLOCK to ESC whereas the 2nd one takes care of CTRL pressed with other keys.

To have this permanently working in X sessions, you can add this to ~/.xprofile.

I went through the answers to this question to do it my own, but I was looking for something slightly different:

1. I'm using Linux
2. I'm happy to change CAPS even outside Vim (I press almost always by mistake, and I can keep it on the Esc button anyhow for the goals I'm using it)
3. I'd like to use the most common software as possible, not to require any further installation
4. I'd prefer to KISS, and then even avoid the need of further lengthy scripts
5. I'd like to have both ABC0 AND CAPS + <something> -> Ctrl + <something>, if possible

Then my current solution it is:

# ~/.xprofile
xmodmap $HOME/.xmodmap

and:

! ~/.xmodmap
clear lock
keycode 66 = Escape
keycode 9 = Caps_Lock
add lock = Caps_Lock


clear control
add control = Control_L Control_R Escape

Looks like it's working like a charm for the time being, but I'm looking for feedbacks :)

With X (Linux), as @Dan Andreatta mentioned, use xmodmap to alter the key mapping, e.g.

xmodmap -e 'clear Lock' -e 'keycode 0x42 = Escape'

Will map Esc to the CapsLock key.

You could also set this in stone by adding it into an X initialization file, such as .xinitrc — which is run when using startx —:

clear lock
keycode 0x42 = Escape

Typically, this is put inside an .Xmodmap file and then it's run inside .xinitrc.

Vim's documentation has an entire page dedicated to this topic, and the comments section is especially helful.