Function to get the text from the visual selection in the vim editor. Vimscript version is compatible in both vim and neovim. Also lua only version.

get_selected_text.lua

-- Function to get the selected text.
-- It handles when current mode is visual mode and when visual mode has ended.
-- It handle visual, visual line and visual block modes
-- The return is a list with the selected text per line from top to bottom and from left to right.
--
-- Based on the answers in the stackoverflow post:
-- https://stackoverflow.com/questions/1533565/how-to-get-visually-selected-text-in-vimscript/28398359
--
-- It has great answers but none quite behave as I expected, so this is my take on it.

-- If you want a lua version, check this up.
-- This is a best effor translation from the vimscript version.
-- If you find an issue or have a suggestion to improve it, feel free to
-- make the suggestion.

---Get the text between the marks a and b using the appropriate mode
---@param a_mark string Reference mark a
---@param b_mark string Reference mark b
---@param mode string Mode to process text from marks
---@return string[] Selected text. One entry per line from left to right.
local get_selected_text_marks = function (a_mark, b_mark, mode)
  local _, line_start, column_start = unpack(vim.fn.getpos(a_mark))
  local _, line_end, column_end = unpack(vim.fn.getpos(b_mark))

  -- Mark could be reversed if starting selection from bottom to top or right to left
  if (vim.fn.line2byte(line_start)+column_start) > (vim.fn.line2byte(line_end)+column_end) then
    line_start, column_start, line_end, column_end = line_end, column_end, line_start, column_start
  end

  -- Should always be an array when passing two arguments
  local lines = vim.fn.getline(line_start, line_end) --[[@as string[] ]]

  -- No selection, return empty
  if #lines == 0 then
    return {}
  end

  -- Handle visual line selection
  if mode == 'V' then
    return lines -- No further process
  end

  -- Handle visual block selection
  if mode == vim.keycode('<C-V>') then
    -- Selection can be reversed if started from right to left
    if column_start > column_end then
      column_start, column_end = column_end, column_start
    end

    if vim.o.selection == 'exclusive' then
      column_end = column_end - 1 -- Needed to remove the last character to make it match the visual selction
    end

    for idx = 1, #lines do
      -- Get just the selected area from each line
      lines[idx] = lines[idx]:sub(1, column_end)
      lines[idx] = lines[idx]:sub(column_start)
    end

    return lines
  end

  -- Handle visual mode 'v'
  if vim.o.selection == 'exclusive' then
    column_end = column_end - 1 -- Needed to remove the last character to make it match the visual selction
  end

  -- Adjust first and last selected lines to the respective start/end position
  lines[#lines] = lines[#lines]:sub(1, column_end)
  lines[1] = lines[1]:sub(column_start)

  return lines
end

---Get text from visual selected area
---@return string[] Selected text. One entry per line from left to right.
local get_selected_text = function ()
  local mode = vim.fn.mode()
  if mode == 'v' or mode == 'V' or mode == vim.keycode('<C-V>') then
    return get_selected_text_marks('v', '.', mode)
  else
    return get_selected_text_marks("'<", "'>", vim.fn.visualmode())
  end
end

get_selected_text.vim

" Function to get the selected text.
" It handles when current mode is visual mode and when visual mode has ended.
" It handle visual, visual line and visual block modes
" The return is a list with the selected text per line from top to bottom and from left to right.
"
" Based on the answers in the stackoverflow post:
" https://stackoverflow.com/questions/1533565/how-to-get-visually-selected-text-in-vimscript/28398359
"
" It has great answers but none quite behave as I expected, so this is my take on it.

" Get selected text from mark positions
function! get_text_from_marks(a_mark, b_mark, mode)
  let mode = a:mode
  let [line_start, column_start] = getpos(a:a_mark)[1:2]
  let [line_end, column_end] = getpos(a:b_mark)[1:2]

  " Mark could be reversed if starting selection from bottom to top or right to left
  if (line2byte(line_start)+column_start) > (line2byte(line_end)+column_end)
    let [line_start, column_start, line_end, column_end] =
      \   [line_end, column_end, line_start, column_start]
  end

  let lines = getline(line_start, line_end)

  " No selection, return empty
  if len(lines) == 0
    return []
  endif

  " Handle visual line selection
  if mode ==# 'V'
    return lines " No further process
  endif

  " Handle visual block selection
  if mode ==# "\<C-V>"
    " Selection can be reversed if started from right to left
    let [column_start, column_end] = column_end > column_start ? [column_start, column_end] : [column_end, column_start]

    if &selection ==# "exclusive"
      let column_end -= 1 " Needed to remove the last character to make it match the visual selction
    endif
    for idx in range(len(lines))
      " Get just the selected area from each line
      let lines[idx] = lines[idx][: column_end - 1]
      let lines[idx] = lines[idx][column_start - 1:]
    endfor

    return lines
  endif

  " Handle visual mode 'v'
  if &selection ==# "exclusive"
    let column_end -= 1 " Needed to remove the last character to make it match the visual selction
  endif

  " Adjust first and last selected lines to the respective start/end position
  let lines[-1] = lines[-1][: column_end - 1]
  let lines[ 0] = lines[ 0][column_start - 1:]

  return lines
endfunction

function! get_selected_text() abort
  " Check current mode
  let curr_mode = mode()
  if curr_mode ==# 'v' || curr_mode ==# 'V' || curr_mode ==# "\<C-V>"
    " This is valid if currently in a type of visual mode trigger by
    " something like `<cmd>call func(get_selected_text())<cr>` while in visual mode
    " We can use positions 'v' (cursor) and '.' (oposite end)
    return get_text_from_marks('v', '.', curr_mode)
  else
    " When current mode is not visual, it means the mode may have ended like from keymaps
    " like `:<C-U>call func(get_selected_text())<cr>` which take you out of visual mode
    " Then the last visual mode can be fetched with visualmode() and positions with the marks '< '>
    return get_text_from_marks("'<", "'>", visualmode())
  end
endfunction