What is the significance of '?ngModel' when creating an AngularJS directive?

Question

I'm working my way through the new ng-book. The chapter on filters includes a section on defining parsers with the following code:

angular.module('myApp')
.directive('oneToTen', function() {
    return {
        require: '?ngModel';

The first time I've seen the '?ngModel' syntax, and the Angular API docs don't provide much help. What does this syntax signify?

Thanks!

Answer 1

? is optional directive and ^ is parent directive

http://docs.angularjs.org/api/ng.$compile

(no prefix) - Locate the required controller on the current element. Throw an error if not found.
? - Attempt to locate the required controller or pass null to the link fn if not found.
^ - Locate the required controller by searching the element's parents. Throw an error if not found.
?^ - Attempt to locate the required controller by searching the element's parents or pass null to the link fn if not found.