Using registers in Neovim Lua

Use the Neovim builtins getreg() and setreg().

Since these are Neovim builtins, you'll need to call them like vim.fn.getreg("0").

local clip = vim.fn.getreg("0")
vim.print(clip)

If setreg is called without a register name, the unnamed register '"' is used. getreg must be called with a register name.

getreg() can be made to return a List rather than a string, if { list = true } in its options:

local shopping_list = vim.fn.getreg("@", { list = true })

To set a register (2) to the value of another register (1):

vim.fn.setreg("2", vim.fn.getreg("1"))

For more, see :h setreg() and :h getreg().