How can I create a constraint to check if an email is valid in postgres?

CREATE TABLE emails (
    email varchar
    CONSTRAINT proper_email CHECK (email ~* '^[A-Za-z0-9._+%-]+@[A-Za-z0-9.-]+[.][A-Za-z]+$')
);

(regex may be incomplete, you can search for regexp for email matching all over the web and pick the one you like best).