How to find and replace string using REGEXP_REPLACE in postgresql

I have a table of email addresses:

CREATE TABLE contacts(
    email     VARCHAR(255)
)

INSERT INTO contacts VALUES    
    ('[e-mail]'),
    ('[e-mail]'),
    ('[e-mail]');

How can I find and replace the email format so [e-mail] -> [e-mail]?

E.g:

UPDATE contacts
SET email = REGEXP_REPLACE(email, '@', '@test.com');

Results in [e-mail]

Playground here: https://dbfiddle.uk/GnIfomiO

Answer

This is probably most simply done by splitting the email address in two on the @, keeping the part before it and replacing . in the part after it with nothing. Then you can just append @test.com to the result:

UPDATE contacts
SET email = SPLIT_PART(email, '@', 1) || '_' || REPLACE(SPLIT_PART(email, '@', 2), '.', '') || '@test.com';

Output for your demo:

email
[e-mail]
[e-mail]
[e-mail]

Demo on dbfiddle