$.validator.addMethod(
     'rule-name',
         function(value, element) {
             // Return true or false after validation rule check
         },
      $.mage.__('Error message to display if validation fails')
)

var config = { 
      config: {
            mixins: {
                 'mage/validation': {
                       'Vendor_Module/js/validation-mixin': true
                 }
          }
   }
}

    define(['jquery'], function($) {
    'use strict';
      return function() {
           $.validator.addMethod(
             'validate-five-words',
                 function(value, element) {
                   return value.split(' ').length == 5;
                },
            $.mage.__('Please enter exactly five words')
         )
      }
});

After this,

Step 3:  Add validation class in the input field

add validate-five-words in the input field as class name

 <input name="title" id="title" title="title" value="" class="input-text validate-five-words" type="text"
>

Step 4: Apply the Changes

Then remove folders present inside pub/static/frontend and var/view_preprocessed and run the below command to see your validation working –

php bin/magento se:s:d -f && php bin/magento c:f

Add More Validations:

You can add more validations also in app/code/Vendor/Module/view/frontend/web/js/validation-mixin.js like this:-

 define(['jquery'], function($) {
    'use strict';
        return function() {
            $.validator.addMethod(
                 'validate-five-words',
                     function(value, element) {
                           return value.split(' ').length == 5;
                     },
                     $.mage.__('Please enter exactly five words')
             ),
             $.validator.addMethod(
                 'validate-seven-length',
                     function(value, element) {
                          return value.length == 7;
                     },
                    $.mage.__('Please enter exactly 7 letters')
            ),
             $.validator.addMethod(
                 'validate-email-abc',
                      function(value, element) {
                          return value.search('@abc.com') != -1;
                      }, 
                      $.mage.__('Please enter @abc.com')
            )
       }
});

Conclusion

In this article, we have learnt about the where are the validation mentioned, what are the ways to create validation, how to create validations and also how to create custom rules for validation on the form in Magento2.

Feel free to post comments, I will definitely read.