Angular ng-pattern. How to check the data?

Question

Angular

Help solve the problem, please.
To begin with, I want to set a condition: all English letters.
From what I read in the documentation https://docs.angularjs.org/api/ng/directive/ngPattern
I don't quite understand why the pattern looks like this?
$scope.regex = '\\d+';
and not like in pure JS?
$scope.regex = '/\d+/';
My code

<form method="post" name="signupForm" ng-submit="register()" role="form">
  <input ng-pattern="username" type="text" name="displayName" ng-model="user.fullName" placeholder="Full Name" required>
</form>

$scope.username = "/^[a-zA-Z]*$/";
If I substitute "/^[a-zA-Z]*$/" directly into ng-pattern it works fine

Answer 1

The code is incomplete, so offhand:
$scope.username = /^[a-zA-Z]{5,25}$/;