<form>
  <label>Enter phone number (e.g. (123) 456-7890): <input type="text" name="phone"></label>
  <input type="submit">
</form>

<p id="result"></p>

* {
  font-family: sans-serif;
}

input,
button {
  font-size: 100%;
  padding: 5px;
}

var result = document.querySelector('#result');
var form = document.querySelector('form');

form.addEventListener('submit', function(e) {
  e.preventDefault();
  checkPhoneNumber(this.phone.value);
});

function checkPhoneNumber(phoneNo) {
  var phoneRE = /^\(\d\d\d\) \d\d\d-\d\d\d\d$/;
  if (phoneNo.match(phoneRE)) {
    result.innerHTML = 'The phone number is <strong>valid!</strong>';
  } else {
    result.innerHTML = 'The phone number is <strong>invalid!</strong>';
  }
}