To chase rainbows

To try to achieve something that will never happen

You can use this phrase in ………… language.

 Spoken
Informal

Example 1
I think my parents never truly believed that I could make it as a professional athlete. They supported me in every possible way, but I’d say they thought I was chasing rainbows.

Example 2
I wonder when he stops chasing rainbows and realizes that he needs to find a job with a stable income, move out from his parents’ house and start living a life.