When working with dynamic regular expressions in AngularJS, especially with ng-pattern, you might encounter issues because ng-pattern expects a static regular expression or a string that represents a regular expression. AngularJS doesn’t automatically interpret a dynamic expression passed into the ng-pattern attribute, which can cause unexpected behavior. To resolve this, there are certain best practices and techniques that you should follow.
1. Understanding ng-pattern
In AngularJS, ng-pattern is a directive that provides pattern-based validation for form inputs. It allows you to bind a regular expression to an input field to validate the entered value. For example, if you want to validate that a user input is a valid email address, you can use:
<input type="text" ng-model="email" ng-pattern="/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/">
Here, ng-pattern validates the email field with the provided regular expression. AngularJS will validate the value and mark the field as valid or invalid based on the regular expression.
2. The Problem with Dynamic Regular Expressions
In some cases, you might want to dynamically generate the regular expression. This could be based on user input, some server-side data, or even other UI interactions. However, AngularJS does not automatically update the ng-pattern directive if you change the regular expression dynamically. This happens because AngularJS expects a static regular expression or a string to be bound to ng-pattern.
For example, consider a situation where you want to dynamically generate a regular expression based on a user’s selected country to validate a phone number:
<input type="text" ng-model="phone" ng-pattern="phonePattern">
Here, phonePattern would need to be dynamically set based on the user’s selection. However, if the pattern is updated programmatically (e.g., through a controller or directive), AngularJS won’t update the ng-pattern validation automatically. This is the problem we need to address.
3. Why ng-pattern Does Not React to Dynamic Changes
AngularJS’s ng-pattern uses the regular expression object itself and doesn’t listen for changes to it. Since AngularJS only listens to changes in the expression or model values and not the regex pattern, it won’t trigger validation updates when the regular expression is updated. Therefore, you need to ensure that the pattern is properly bound and updated dynamically.
4. Workaround 1: Using $watch to Bind Dynamic Patterns
One way to dynamically update the regular expression for ng-pattern is by using $watch. The $watch function allows you to monitor changes to a specific scope variable (in this case, the regular expression) and take action when it changes.
Here’s how you can use $watch to update the pattern dynamically:
app.controller('PhoneController', function($scope) {
$scope.countries = ['USA', 'India', 'UK'];
$scope.selectedCountry = 'USA';
// Watch for changes in the selected country
$scope.$watch('selectedCountry', function(newVal, oldVal) {
if (newVal === 'USA') {
$scope.phonePattern = /^[2-9]{1}[0-9]{9}$/; // Valid USA phone number
} else if (newVal === 'India') {
$scope.phonePattern = /^[7-9]{1}[0-9]{9}$/; // Valid India phone number
} else if (newVal === 'UK') {
$scope.phonePattern = /^[0-9]{10}$/; // Valid UK phone number
}
});
});
And in your HTML:
<select ng-model="selectedCountry" ng-options="country for country in countries"></select>
<input type="text" ng-model="phone" ng-pattern="phonePattern">
In this example, the $watch function listens for changes to the selectedCountry and updates the regular expression accordingly. This ensures that when a user selects a different country, the phonePattern is updated, and AngularJS will validate the phone number based on the new pattern.
5. Workaround 2: Using ng-model-options with $parsers
Another solution is to use ng-model-options with $parsers. $parsers are used to transform input values before they are passed to the model. You can modify the input’s validation behavior by adding a custom parser that applies the dynamic regular expression.
Here’s an example of how to use this approach:
<input type="text" ng-model="phone" ng-model-options="{updateOn: 'blur'}" ng-pattern="phonePattern">
In your controller, you can update the pattern dynamically and use $parsers to intercept the input:
app.controller('PhoneController', function($scope) {
$scope.countries = ['USA', 'India', 'UK'];
$scope.selectedCountry = 'USA';
$scope.$watch('selectedCountry', function(newVal) {
if (newVal === 'USA') {
$scope.phonePattern = /^[2-9]{1}[0-9]{9}$/; // Valid USA phone number
} else if (newVal === 'India') {
$scope.phonePattern = /^[7-9]{1}[0-9]{9}$/; // Valid India phone number
} else if (newVal === 'UK') {
$scope.phonePattern = /^[0-9]{10}$/; // Valid UK phone number
}
});
$scope.$watch('phone', function(newPhone) {
// Custom $parsers logic here to dynamically validate
if ($scope.phonePattern && !newPhone.match($scope.phonePattern)) {
$scope.phone.$setValidity('pattern', false);
} else {
$scope.phone.$setValidity('pattern', true);
}
});
});
6. Workaround 3: Using Custom Directives
If neither of the above methods works for your use case, you can create a custom directive that allows dynamic regular expressions. With this approach, you have full control over how the regular expression is applied.
Here’s an example of a custom directive to handle dynamic patterns:
app.directive('dynamicPattern', function() {
return {
restrict: 'A',
require: 'ngModel',
scope: {
dynamicPattern: '='
},
link: function(scope, element, attrs, ngModel) {
ngModel.$validators.pattern = function(modelValue, viewValue) {
var value = modelValue || viewValue;
if (value) {
var regex = new RegExp(scope.dynamicPattern);
return regex.test(value);
}
return true;
};
}
};
});
In your HTML:
<input type="text" ng-model="phone" dynamic-pattern="phonePattern">
This custom directive allows you to pass a dynamic regular expression as dynamic-pattern. It watches for changes to the regular expression and applies it to the model validation.
7. Best Practices for Dynamic Regular Expressions
- Avoid using
ng-patterndirectly with dynamic expressions: As seen, AngularJS has limited built-in support for dynamic regular expressions. Instead, use custom solutions like$watchor custom directives. - Test your regular expressions thoroughly: When generating regular expressions dynamically, ensure they are valid and don’t lead to unexpected behavior. Use regex testing tools to verify your expressions before implementing them.
- Optimize performance: If you’re watching multiple dynamic expressions or using
$watchon many variables, make sure to optimize your code to prevent performance issues.