Alternating files not working in nvim script but does in vimscript

[kapral18]

Could anybody please kindly point me to why this works:

-- Remap Ctrl-^ to switch between alternate test and source files
vim.api.nvim_set_keymap("n", "<C-^>", ":call SwitchAlternateFile()<CR>", { noremap = true, silent = true })

vim.api.nvim_exec(
  [[
  function! SwitchAlternateFile()
    let current_file = expand("%")
    let alternate_file = substitute(
      \ current_file,
      \ '\v(\w+)(\.spec|\.test)?(\.\w+)$',
      \ '\=submatch(1) . (empty(submatch(2)) ? GetFileExtension() : "") . submatch(3)',
      \ ""
    \)
    if alternate_file != "" 
      execute "e " . alternate_file
    else
      normal! <C-^>
    endif
  endfunction

  function! GetFileExtension()
    if filereadable(resolve(expand("%:.:h") . "/" . submatch(1) . ".spec" . submatch(3)))
      return ".spec"
    elseif filereadable(resolve(expand("%:.:h") . "/" . submatch(1) . ".test" . submatch(3)))
      return ".test"
    else
      return ""
    endif
  endfunction
]],
  false
)

but the same thing in pure lua doesn't

-- Remap Ctrl-^ to switch between alternate test and source files
vim.api.nvim_set_keymap("n", "<C-^>", ":lua SwitchAlternateFile()<CR>", { noremap = true, silent = true })

function SwitchAlternateFile()
  local current_file = vim.fn.expand("%")
  local alternate_file = vim.fn.substitute(
    current_file,
    [[\v(\w+)(\.spec|\.test)?(\.\w+)$]],
    [[\=submatch(1) ..
      (function()
        local spec_file_path = expand("%:.:h") .. "/" .. submatch(1) .. ".spec" .. submatch(3)
        local test_file_path = expand("%:.:h") .. "/" .. submatch(1) .. ".test" .. submatch(3)
        if filereadable(resolve(spec_file_path)) then
          return ".spec"
        elseif filereadable(resolve(test_file_path)) then
          return ".test"
        else
          return ""
        end
      end)() .. submatch(3)
    ]],
    ""
  )
  if alternate_file ~= "" then
    vim.cmd("e " .. alternate_file)
  else
    vim.cmd("normal! <C-^>")
  end
end

fails with invalid lua vim syntax error

[zeertzjq]

You cannot use Lua syntax inside substitute expression.

[kapral18]

but how come [[\=submatch(1) .. submatch(3)]] works?

[zeertzjq]

That's valid Vimscript.

[kapral18]

@zeertzjq my bad. Thanks for clarifying.