<form id="register" action="" method="get">
  <input type="text" id="phone" name="the-phone" maxlength="15" />
  <input type="submit" value="Register" />
</form>

/*
  Regular Expression
  - Input Form Validation Practice
*/

document.getElementById("register").onsubmit = function () {
  let phoneInput = document.getElementById("phone").value;
  let phoneRe = /(d{4})sd{3}-d{4}/; // (1234) 567-8910
  let validationResult = phoneRe.test(phoneInput);
  if (validationResult === false) {
    return false;
  }
  return true;
}