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">
    <form name="form" ng-submit="submit()" novalidate>
        <input type="text" ng-model="firstname_lastname"  size="30" ng-pattern="/^(.*?[a-zA-Z]){2,}$/" placeholder="add new todo here" required=""/>
        
        <button type="submit">Submit</button>
    </form>
  </div>
</div>

CSS

.done-true {
  text-decoration: line-through;
  color: grey;
}

JavaScript

function TodoCtrl($scope) {
    $scope.submit = function(){
        if($scope.form.$invalid) //here you can stop use from submiting for
            alert('Form is invalid'); //form is invalid
        else
             alert('Form is valid');//here you can do actual submit to server
    }
}