How to get visually selected text in VimScript

The best way I found was to paste the selection into a register:

function! lh#visual#selection()
  try
    let a_save = @a
    normal! gv"ay
    return @a
  finally
    let @a = a_save
  endtry
endfunction

I came here asking the same question as the topic starter and tried the code by Luc Hermitte but it didn't work for me (when the visual selection is still in effect while my code is executed) so I wrote the function below, which seems to work okay:

function! s:get_visual_selection()
    let [line_start, column_start] = getpos("'<")[1:2]
    let [line_end, column_end] = getpos("'>")[1:2]
    let lines = getline(line_start, line_end)
    if len(lines) == 0
        return ''
    endif
    let lines[-1] = lines[-1][: column_end - 2]
    let lines[0] = lines[0][column_start - 1:]
    return join(lines, "\n")
endfunction

I hope this is useful to someone!

Update (May 2013): Actually that's not quite correct yet, I recently fixed the following bug in one of the Vim plug-ins I published:

function! s:get_visual_selection()
    " Why is this not a built-in Vim script function?!
    let [line_start, column_start] = getpos("'<")[1:2]
    let [line_end, column_end] = getpos("'>")[1:2]
    let lines = getline(line_start, line_end)
    if len(lines) == 0
        return ''
    endif
    let lines[-1] = lines[-1][: column_end - (&selection == 'inclusive' ? 1 : 2)]
    let lines[0] = lines[0][column_start - 1:]
    return join(lines, "\n")
endfunction

Update (May 2014): This (trivial) code is hereby licensed as public domain. Do with it what you want. Credits are appreciated but not required.

On Linux, there is a cheap but effective alternative to programming such a GetVisualSelection() function yourself: use the * register!

The * register contains the content of the most recent Visual selection. See :h x11-selection.

In your script you could then simply access @* to get the Visual selection.

let v = @*

Incidentally, * is also a neat little helper in interactive use. For example, in insert mode you can use CTRL-R * to insert what you had selected earlier. No explicit yanking involved.

This works only on operating systems that support the X11 selection mechanism.