func! RegionMakeBold()
  hi MyBold gui=bold
  let l_start = getpos("'<")[1]
  let l_end = getpos("'>")[1]
  let c_start = getpos("'<")[2]
  let c_end = getpos("'>")[2]+1
  execute 'syntax region MyBold start=/%'.l_start.'l%'.c_start.'c/ end=/%'.l_end.'l%'.c_end.'c/'
endfunc

vnoremap <leader>b :<C-U>call RegionMakeBold()<CR>