Validate phone number with JavaScript

Question

I found this code in some website, and it works perfectly. It validates that the phone number is in one of these formats:
(123) 456-7890 or 123-

Answer 1

What I would do is ignore the format and validate the numeric content:

var originalPhoneNumber = "415-555-1212";

function isValid(p) {
  var phoneRe = /^[2-9]\d{2}[2-9]\d{2}\d{4}$/;
  var digits = p.replace(/\D/g, "");
  return phoneRe.test(digits);
}