Define custom commands for the * and + registers

Question

If I'm using a build of Vim with the xterm_clipboard feature then the system clipboard content is available through the + register.

If the build I'm using doesn't have this feature, can I make the "+ register call my custom programs to retrieve and set the system clipboard (xsel -bo and xsel -bi, or likewise with xclip)? Likewise with "* to call xsel -po and xsel -pi.

The point is to be able to type something like "+p without having to worry whether the feature is available. Typing :r !xsel -b<Enter> is not what I'm looking for: I want the same keystrokes to work, and I want the paste variants to follow what's available for “true” registers (P, gp, …).

Bonus: can I define custom commands for other register names?

Answer 1

A few <expr> mapping should be able to do the job here. The basic structure would be

function! ClipboardOrXclip(command, register)
    if a:register !~ '[+*]' || has('xterm_clipboard') || has('gui_running')
        " Just return the original command if the clipboard is accessible
        " or it's not a register that should be handled by xsel
        return a:command
    endif
    if a:register == '+'
        return "<Esc>:r !xsel -bo<CR>"
    else
        return "<Esc>:r !xsel -po<CR>"
    endif
endfunction

nnoremap <silent> <expr> p ClipboardOrXclip('p', v:register)

The function would need to be expanded to handle all the variations of p, P, gp, etc. but this should be a good start.