How can I read the contents of a buffer or line then rewrite them, from a Lua script?

Question

I am trying to write a pretty-printer Lua plugin for a LaTeX-like language. The idea is that if a user types \cup in insert mode then presses <Space> or <Tab> or <CR>, then my script will read the preceding string and transform it into ∪. So far I have:

_G.rewrite_unicode = function(keystroke)
  local r, c = unpack(vim.api.nvim_win_get_cursor(0))
  return keystroke
end

vim.api.nvim_set_keymap('i', '<Space>', "v:lua.rewrite_unicode('<Space>')", { noremap = true, expr = true })
vim.api.nvim_set_keymap('i', '<Tab>', "v:lua.rewrite_unicode('<Tab>')", { noremap = true, expr = true })
vim.api.nvim_set_keymap('i', '<CR>', "v:lua.rewrite_unicode('<CR>')", { noremap = true, expr = true }

but I don't know how I can read the string preceding the cursor position in the buffer, then rewrite it. What APIs would I use for that?

Answer 1

A far simpler version of what you're trying to accomplish:

inoreabrrev <buffer> \cup ∪

To read/write a buffer, you probably want the API at :help text-functions.

If you meant register, you can access it directly @a to read/write it.

If you need to do "smarter" things, I would look for a snippets plugin.