AngularJS Example:

HTML

<link rel="stylesheet" href="http://twitter.github.com/bootstrap/assets/css/bootstrap.css">
<div ng-app>
  <h2>Todo</h2>
  <div ng-controller="TodoCtrl">
      <input type="text" ng-model="searchText"/>
      <div>Hello</div>
      <div ng-show="show">World !</div>
  </div>
</div>

JavaScript

function TodoCtrl($scope) {
    var reg = /20\d{2}/g;    
    var isValid = function(str) {
        $scope.show = reg.test(str);
    }
    $scope.$watch('searchText',isValid);

}