vim backups in .vimrc

set backup

function! BackupDir()
   if has('win32') || has('win64')
      let l:backupdir=$VIM.'/backup/'.
               \substitute(expand('%:p:h'), '\:', '~', '')
   else
      let l:backupdir=$HOME.'/.vim/backup/'.
               \substitute(expand('%:p:h'), '^'.$HOME, '~', '')
   endif

   if !isdirectory(l:backupdir)
      call mkdir(l:backupdir, 'p', 0700)
   endif

   let &backupdir=l:backupdir
   let &backupext=strftime('~%Y-%m-%d_%H-%M-%S~')
endfunction

autocmd! bufwritepre * call BackupDir()