Regular expressions (Regex) look like absolute gibberish to beginners, but they are arguably the most powerful text-processing tool in a developer's arsenal.
Client vs Server Validation
While you should always use Regex to validate emails and phone numbers on the frontend for UX, you must repeat that exact same Regex validation on your backend to prevent malicious injections.
Testing your Regex patterns thoroughly is vital, as a poorly written Regex can lead to catastrophic backtracking (ReDoS attacks).