How to rewrite regex for vim?

Question

vim

There is a regex that looks for haskell function names, we need to rewrite it for vim.
Original expression:
^[az]+[ \w | ']\s*(?=::)
vim expression:
/^[az]\+[\w']*\s*\(::\)@=/
But unlike the original one, vim expression for some reason does not work

Answer 1

^[a-z_]\(\S\&[^,\(\)\[\]]\)*
took from here https://github.com/sdiehl/haskell-vim-proto/blob/m...
Maybe it's better to use a ready-made plugin? the same haskell-vim