regex in RE2 syntax with Negate match

Question

How can I write a regex in RE2 syntax to match strings which contain 'register' but doesn't contain 'dashboard'?

E.g.

• https://domain.co/auth/login - NOT

• https://domain.co/auth/register?emailgfdg%40g.com&token=5F6DCCA1 - OK

• https://domain.co/auth/register?emailgfdg%40g.com&dashboardtoken=5F6DCCA1 - NOT

PS 'dashboard' (if any) is always placed after 'register'

I'd use .*register(?!.*dashboard).* but RE2 doesn't support that syntax.