Magento 2 : Problem Adding Validation Form With Mixin

@Logan great info, there is a more supported way to add to the UI rules.

Add the mixin:

'Magento_Ui/js/lib/validation/validator': {
            'js/my-ui-validation': true
        }

Then add the rule definition:

define([
    'jquery',
], function ($) {
    'use strict';

    return function (validator) {

        validator.addRule(
            'my-custom-phone-number-rule',
            function (value, params, additionalParams) {
                return $.mage.isEmptyNoTrim(value) || value.match(/^[0-9 .-]+$/);
            },
            $.mage.__("Please enter a valid phone number.")
        );

        return validator;
    };
});

Hi @Jurģis Toms Liepiņš - your answer helped me a lot!

Multiple Validation Methods

It appears there are two JavaScript validation implementations:

1. mage/validation.js

The mage/validation file is referenced by mage/validation/validation, which is mapped with RequireJS to validation:

'validation': 'mage/validation/validation'

Inchoo has an article on how to use this.

2. Magento_Ui/js/lib/validation/rules.js

The Magento_Ui/js/lib/validation/rules.js file is referenced in Magento_Ui/js/form/element/abstract and Magento_Ui/js/form/element/file-uploader and seems to be used to validate UI form elements.

Example using this validation in XML layout:

<item name="example-input-element" xsi:type="array">
    <item name="component" xsi:type="string">Magento_Ui/js/form/element/abstract</item>
    <item name="config" xsi:type="array">
        <item name="customScope" xsi:type="string">shippingAddress</item>
        <item name="template" xsi:type="string">ui/form/field</item>
        <item name=elementTmpl" xsi:type="string">ui/form/element/input</item>
    </item>
    <item name="validation" xsi:type="array">
        <item name="required-entry" xsi:type="boolean">true</item>
        <item name="max_text_length" xsi:type="number">255</item>
        <item name="validate-address-not-pobox" xsi:type="boolean">true</item>
    </item>
</item>

---

Usage + Examples

Here is the necessary code to add custom validation rules to the two implementations:

Example RequireJS Configuration

/**
 * File:
 * requirejs-config.js
 */

var config = {
    config: {
        mixins: {
            'mage/validation': {
                'js/my-validation': true
            },
            'Magento_Ui/js/lib/validation/rules': {
                'js/my-ui-validation': true
            }
        }
    }
};

Example mage/validation Mixin

/**
 * File:
 * js/my-validation.js
 */

define(['jquery'], function ($) {
    'use strict';
    
    return function() {
        // PO Box addresses
        $.validator.addMethod(
            'validate-address-not-pobox',
            function (v) {
                return !/p(s|.){0,2}?o(s|.){0,2}?.*box|post(s|.){0,2}?office/i.test(v);
            },
            $.mage.__('Address cannot be a PO Box address.')
        );
    }
});

Example Magento_Ui/js/lib/validation/rules Mixin

Note: Joel Davey shows another great way to add validation methods for UI elements

/**
 * File:
 * js/my-ui-validation.js
 */

define(['mage/translate'], function($t) {
    'use strict';
    
    return function(rules) {
        rules['validate-address-not-pobox'] = {
            handler: function (v) {
                return !/p(s|.){0,2}?o(s|.){0,2}?.*box|post(s|.){0,2}?office/i.test(v);
            },
            message: $t('Address cannot be a PO Box address.')
        };
        return rules;
    };
});