---
title: "Custom validation rules and validation messages in knockout"
description: "Implement custom validation rules and validation messages in Knockout.js to enhance form validation. This guide covers setting up Knockout Validation."
author: "ICSM Computer"
published: 2024-05-27
updated: 2024-05-27
canonical: https://www.mindstick.com/articles/335925/custom-validation-rules-and-validation-messages-in-knockout
category: "knockcout js"
tags: ["javascript", "knockout.js", "front-end development"]
reading_time: 5 minutes
---
# Custom validation rules and validation messages in knockout
In Knockout, you can create [custom validation](https://www.mindstick.com/forum/1973/custom-validation-attribute) rules and messages by using the knockout-[validation plugin](https://www.mindstick.com/forum/157975/how-can-use-jquery-s-form-validation-plugin-to-validate-user-input-and-provide-feedback-to-users).
Here's an example of how you can create a custom [validation rule](https://answers.mindstick.com/qa/116054/can-you-create-fack-email-validation-rule) and message:
**First**, include the knockout-validation plugin in [your project](https://answers.mindstick.com/qa/93843/is-it-necessary-to-add-bootstrap-js-and-bootstrap-css-both-in-your-project) by adding the following script tag in your HTML file:
```html
```
**Next**, create a custom validation rule and message. For example, let's say we want to create a custom validation rule that checks if the input value is a valid [email address](https://www.mindstick.com/forum/160829/how-can-i-update-my-email-address-to-log-in-to-mindstick-training):
```html
ko.validation.rules['email'] = {
validator: function (value, params) {
return /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(value);
},
message: 'Please enter a valid email address'
};
// Apply the custom validation rule to the observable
ko.validation.registerExtenders();
```
**Finally**, use the custom validation rule in your HTML markup like this:
```html
```
Now, when a user enters an invalid email address, the custom validation message 'Please enter a valid email address' will be displayed below the [input field](https://www.mindstick.com/forum/160748/how-to-set-focus-on-input-field-in-angularjs).
By following these steps, you can create custom [validation rules](https://www.mindstick.com/interview/34052/custom-validation-rules-and-validation-messages-in-knockout) and messages in Knockout using the knockout-validation plugin. This allows you to customize the validation logic and [error messages](https://answers.mindstick.com/qa/99815/how-can-you-diagnose-and-resolve-issues-with-application-crashes-or-error-messages) to fit your specific requirements.
## Example -
```html
Bootstrap demo