noremap s :set opfunc=SensibleSubstitute<CR>g@
function! SensibleSubstitute(type)
    normal `[v`]
    normal "_c<C-R>"
endfunction

exec 'normal "_c'."<C-r>".'"'

noremap s :set opfunc=SensibleSubstitute<CR>g@
function! SensibleSubstitute(type)
  echo v:register
endfunction

"a:set opfunc=SensibleSubstitute<CR>g@

:set opfunc=SensibleSubstitute<CR>"ag@

noremap <expr> s ":set opfunc=SensibleSubstitute<CR>".'"'.v:register."g@"
function! SensibleSubstitute(type)
  normal `[v`]
  exec 'normal "_c'."<C-r>".v:register
endfunction