Prefer server side validations rather than client side

Recently, my team developed server side validations for accepting a world wide phone number with regular expressions. Unfortunately, it was not well tested on the server and our customer were getting errors while saving data. We were able to see these errors in our server logs

Had this validation was done on UI client side, we would not have figured out what is happening with customers and we would not have corrected it quickly with our suffering huge customer loss

Lesson: If you don't have UI testing team, push validations to the server side even if it is not a good ui experience.