title: HTML phone number pattern in <input>
body: <p>I'm working on a HTML form and and I have this input tag for a phone number. 
  I would like the phone numbers inserted in the textbox to match the following format: +389-xxx-xxx , such as e.g. +389-111-222 .
  but I've struggled so far to make the pattern attribute.
  Can someone give me some suggestions to validate the input using only HTML5? Tnx &lt;3</p>