Angular: Empty Fiddle

http://angularjs.org/

HTML

<script src="http://code.angularjs.org/angular-1.0.1.js"></script>
<input phone-format /><span class='helpblock'></span>

JavaScript

var myApp = angular.module('myApp',[]);

//myApp.directive('myDirective', function() {});
//myApp.factory('myService', function() {});

 myApp.directive("phoneFormat", function () {
     
    return {
        restrict: "A",
        link: function (scope, element, attr) {
           
            element.bind('change', function() {
                if ( this.value.length === 10 ) {
                   var number = this.value
                   this.value = number.substring(0,3) + '-' + number.substring(3,6) + '-' + number.substring(6,10)
                }
                else {
                    document.querySelector('.helpblock').innerHTML = 'error in formatting';
                    
                }
            });
           
        }
    };
                     
});