Alexander-Pop/checkout_index_index.xml

## checkout_index_index.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="checkout" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceBlock name="checkout.root">
            <arguments>
                <argument name="jsLayout" xsi:type="array">
                    <item name="components" xsi:type="array">
                        <item name="checkout" xsi:type="array">
                                <item name="steps" xsi:type="array">
                                    <item name="children" xsi:type="array">
                                        <item name="shipping-step" xsi:type="array">
                                            <item name="children" xsi:type="array">
                                                <item name="shippingAddress" xsi:type="array">
                                                    <item name="children" xsi:type="array">
                                                        <item name="shipping-address-fieldset" xsi:type="array">
                                                            <item name="children" xsi:type="array">
                                                                <item name="city" xsi:type="array">
                                                                    <item name="config" xsi:type="array">
                                                                        <item name="validation" xsi:type="array">
                                                                           <item name="validate-alpha-with-spaces" xsi:type="boolean">true</item>
                                                                        </item>
                                                                    </item>
                                                                </item>
                                                            </item>
                                                        </item>
                                                </item>
                                            </item>
                                        </item>
                                    </item>
                                </item>
                            </item>
                        </item>
                    </item>
                </argument>
            </arguments>
        </referenceBlock>
    </body>
</page>

## examples.js
var rules = {
        'validate-min-letters':[
            function (value, element, params) {
                return this.optional(element) || /^[a-zA-Z \-]+$/i.test(value) && value.length >= params;
            },
            'Please enter at least {0} letter without numeric character.' //{0} will be replaced by params
        ],

        /** In case where you need to translate the error message **/
          'validate-min-letters':[
                function (value, element, params) {
                    this.params = params;
                    return this.optional(element) || /^[a-zA-Z \-]+$/i.test(value) && value.length >= params;
                }, function() {
                    return $.mage.__('Please enter at least %1 letters').replace('%1' , this.params)
                }
            ],

        'validate-min-character':[
                function (value, element, params) {
                   this.params = params;
                    return this.optional(element) || /^(.*)+$/i.test(value) && value.length >= params;
                }, function() {
                    return $.mage.__('Please enter at least %1 characters').replace('%1' , this.params);
                }
            ],

        'validate-zip-fr':[
             function(v){
                 return $.mage.isEmptyNoTrim(v) || /(^\d{5}$)/.test(v);
             },
             $.mage.__('Please enter a valid fr zip code.')
        ],

        'validate-alpha-with-spaces:[
            function (v) {
                return $.mage.isEmptyNoTrim(v) || /^[a-zA-Z ]+$/.test(v);
            },
            $.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
        ],
}

## form.phtml
<form action="<?php $block->getFormAction();?>">
  <!-- Here we want at least 2 letter -->
  <input name="lastname" id="lastname" title="<?php echo __('Name') ?>" class="input-text" type="text" data-validate="{'validate-min-letters':2, required:true}"/>

  <!-- Here we want at 5 numeric character -->
  <input type="text" name="postcode" title="<?php echo __('Zip/Postal Code') ?>" id="zip" class="input-text validate-zip-fr"/>

  <button type="submit" title="<?php echo __('Submit') ?>">
      <span><?php echo __('Submit') ?></span>
   </button>
</form>

## my-validation-mixin.js
/** Vendor/Module/view/frontend/web/js/validation-mixin.js */

define([
    'jquery',
    'Magento_Ui/js/lib/validation/validator'
], function ($, validator) {
    'use strict';

    return function () {
        $.validator.addMethod(
            'validate-alpha-with-spaces',
            function (v) {
                return $.mage.isEmptyNoTrim(v) || /^[a-zA-Z ]+$/.test(v);
            },
            $.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
        );


      /** validator addRule method allow to use our custom validation with ui component field (checkout) */

        validator.addRule(
            'validate-alpha-with-spaces',
            function (v) {
                return $.mage.isEmptyNoTrim(v) || /^[a-zA-Z ]+$/.test(v);
            },
            $.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
        );
    };
});

## requirejs-config.js
/** Vendor/Module/view/frontend/requirejs-config.js */

var config = {
  config: {
    mixins: {
      'mage/validation': {
        'Vendor_Module/js/my-validation-mixin': true,
      }
    }
  }
};
	<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="checkout" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
	<body>
	<referenceBlock name="checkout.root">
	<arguments>
	<argument name="jsLayout" xsi:type="array">
	<item name="components" xsi:type="array">
	<item name="checkout" xsi:type="array">
	<item name="steps" xsi:type="array">
	<item name="children" xsi:type="array">
	<item name="shipping-step" xsi:type="array">
	<item name="children" xsi:type="array">
	<item name="shippingAddress" xsi:type="array">
	<item name="children" xsi:type="array">
	<item name="shipping-address-fieldset" xsi:type="array">
	<item name="children" xsi:type="array">
	<item name="city" xsi:type="array">
	<item name="config" xsi:type="array">
	<item name="validation" xsi:type="array">
	<item name="validate-alpha-with-spaces" xsi:type="boolean">true</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</item>
	</argument>
	</arguments>
	</referenceBlock>
	</body>
	</page>
	var rules = {
	'validate-min-letters':[
	function (value, element, params) {
	return this.optional(element) \|\| /^[a-zA-Z \-]+$/i.test(value) && value.length >= params;
	},
	'Please enter at least {0} letter without numeric character.' //{0} will be replaced by params
	],

	/ In case where you need to translate the error message /
	'validate-min-letters':[
	function (value, element, params) {
	this.params = params;
	return this.optional(element) \|\| /^[a-zA-Z \-]+$/i.test(value) && value.length >= params;
	}, function() {
	return $.mage.__('Please enter at least %1 letters').replace('%1' , this.params)
	}
	],

	'validate-min-character':[
	function (value, element, params) {
	this.params = params;
	return this.optional(element) \|\| /^(.*)+$/i.test(value) && value.length >= params;
	}, function() {
	return $.mage.__('Please enter at least %1 characters').replace('%1' , this.params);
	}
	],

	'validate-zip-fr':[
	function(v){
	return $.mage.isEmptyNoTrim(v) \|\| /(^\d{5}$)/.test(v);
	},
	$.mage.__('Please enter a valid fr zip code.')
	],

	'validate-alpha-with-spaces:[
	function (v) {
	return $.mage.isEmptyNoTrim(v) \|\| /^[a-zA-Z ]+$/.test(v);
	},
	$.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
	],
	}
	<form action="<?php $block->getFormAction();?>">
	<!-- Here we want at least 2 letter -->
	<input name="lastname" id="lastname" title="<?php echo __('Name') ?>" class="input-text" type="text" data-validate="{'validate-min-letters':2, required:true}"/>

	<!-- Here we want at 5 numeric character -->
	<input type="text" name="postcode" title="<?php echo __('Zip/Postal Code') ?>" id="zip" class="input-text validate-zip-fr"/>

	<button type="submit" title="<?php echo __('Submit') ?>">
	<span><?php echo __('Submit') ?></span>
	</button>
	</form>
	/** Vendor/Module/view/frontend/web/js/validation-mixin.js */

	define([
	'jquery',
	'Magento_Ui/js/lib/validation/validator'
	], function ($, validator) {
	'use strict';

	return function () {
	$.validator.addMethod(
	'validate-alpha-with-spaces',
	function (v) {
	return $.mage.isEmptyNoTrim(v) \|\| /^[a-zA-Z ]+$/.test(v);
	},
	$.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
	);


	/** validator addRule method allow to use our custom validation with ui component field (checkout) */

	validator.addRule(
	'validate-alpha-with-spaces',
	function (v) {
	return $.mage.isEmptyNoTrim(v) \|\| /^[a-zA-Z ]+$/.test(v);
	},
	$.mage.__('Please use only letters (a-z or A-Z) or spaces only in this field.')
	);
	};
	});
	/** Vendor/Module/view/frontend/requirejs-config.js */

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