Angular pattern doesn't work

Question

I have a problem with ng-pattern.

function formCtrl($scope){}

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app ng-controller="formCtrl">
  <form name="myForm">
    <input ng-model="sign_username"ng-minlength="6" ng-maxlength="15" ng-pattern="/[\p{L}0-9]+$/u" required>
  </form>
  {{myForm.$valid}}
</div>

ng-minlength and ng-maxlength work correctly but when I use special characters ng-pattern still is positive.

I want to check if field contains only letters (unicode) and digits.