Custom on the fly e-mail alerts

Is there a way where users can opt-in for ‘random keywords’ that match their interests?

The user can just enter any keyword they want in a search box and hit enter.

After that the user will be given the option to opt-in for that specific keyword.

After that if content is posted with keywords in the title that matches that keyword, an email will be generate and sent to that user.

It's much like the way Indeed works.

I'm hoping someone can point me in the right direction.