be chasing rainbows
To be constantly pursuing things that are unrealistic or unlikely to happen. You can't always be chasing rainbows your whole life—you need to pick a stable career and start being an adult.
be chasing rainbows
If someone is chasing rainbows, they are wasting their time by trying to get something which they can never have. My teachers told me I'd never make it as an actor and that I was just chasing rainbows