Magento 2 add custom product attribute validation from install script

One of the solution is to add a backend model to your attribute which is used to format / validate your attribute value before save and/or after load.

Add a backend class :

[
    'type' => 'int',
    'backend' => '\Foo\Bar\Model\Attribute\Backend\YourAttribute',
    'frontend' => '',
    'label' => 'XXXX',
    'input' => 'text',
    'frontend_class' => 'validate-greater-than-zero',
    'source' => '',
    'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
    'visible' => true,
    'required' => true,
    'user_defined' => false,
    'default' => 0,
    'searchable' => false,
    'filterable' => true,
    'comparable' => false,
    'visible_on_front' => false,
    'used_in_product_listing' => true,
    'unique' => false
]

Here is an example of your custom class \Foo\Bar\Model\Attribute\Backend\YourAttribute

<?php

namespace Foo\Bar\Model\Attribute\Backend;

/**
 * Class YourAttribute
 */
class YourAttribute extends \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
{

    /**
     * @var int $minimumValueLength
     */
    protected $minimumValueLength = 0;

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function afterLoad($object)
    {
        // your after load logic

        return parent::afterLoad($object);
    }

    /**
     * @param \Magento\Framework\DataObject $object
     *
     * @return $this
     */
    public function beforeSave($object)
    {
        $this->validateLength($object);

        return parent::beforeSave($object);
    }

    /**
     * Validate length
     *
     * @param \Magento\Framework\DataObject $object
     *
     * @return bool
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function validateLength($object)
    {
        /** @var string $attributeCode */
        $attributeCode = $this->getAttribute()->getAttributeCode();
        /** @var int $value */
        $value = (int)$object->getData($attributeCode);
        /** @var int $minimumValueLength */
        $minimumValueLength = $this->getMinimumValueLength();

        if ($this->getAttribute()->getIsRequired() && $value <= $minimumValueLength) {
            throw new \Magento\Framework\Exception\LocalizedException(
                __('The value of attribute "%1" must be greater than %2', $attributeCode, $minimumValueLength)
            );
        }

        return true;
    }

    /**
     * Get minimum attribute value length
     * 
     * @return int
     */
    public function getMinimumValueLength()
    {
        return $this->minimumValueLength;
    }
}

If you want a simple example of that kind of class you can check

• \Magento\Customer\Model\Customer\Attribute\Backend\Website
• all the classes which extend \Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend
• the classes into backend_model column in eav_attribute table

---

EDIT
If you want a class that do nearly the same thing as you want you can take a look at the SKU attribute validation \Magento\Catalog\Model\Product\Attribute\Backend\Sku
I also added the method in the example class

---

EDIT
Another solution (maybe not the best one) is to create a plugin on the function \Magento\Eav\Helper\Data::getFrontendClasses and add your frontend class here that can be validated in front.

With the help of Matthéo Geoffray, this is what I did to apply frontend validation for custom attributes.

[
    'type' => 'int',
    'backend' => '',
    'frontend' => '',
    'label' => 'XXXX',
    'input' => 'text',
    'frontend_class' => 'validate-greater-than-zero',
    'source' => '',
    'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
    'visible' => true,
    'required' => true,
    'user_defined' => false,
    'default' => 0,
    'searchable' => false,
    'filterable' => true,
    'comparable' => false,
    'visible_on_front' => false,
    'used_in_product_listing' => true,
    'unique' => false
]

This is the custom attribute in install script.

I added plugin in di.xml

<type name="Magento\Catalog\Ui\DataProvider\CatalogEavValidationRules">
      <plugin name="namespace_custom_validation_for_product_attribute" type="Namespace\Module\Model\Plugin\Product\ValidationRules"/>
</type>

Here is the plugin code.

<?php

namespace Namespace\Module\Model\Plugin\Product;

use Closure;

class ValidationRules
{

    /**
     * @param \Magento\Catalog\Ui\DataProvider\CatalogEavValidationRules $rulesObject
     * @param callable $proceed
     * @param \Magento\Catalog\Api\Data\ProductAttributeInterface $attribute,
     * @param array $data
     * @return array
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function aroundBuild(
        \Magento\Catalog\Ui\DataProvider\CatalogEavValidationRules $rulesObject,
        Closure $proceed,
        \Magento\Catalog\Api\Data\ProductAttributeInterface $attribute,
        array $data
    ){
        $rules = $proceed($attribute,$data);
        if($attribute->getAttributeCode() == 'xyz'){ //custom filter
            $validationClasses = explode(' ', $attribute->getFrontendClass());
            foreach ($validationClasses as $class) {
                $rules[$class] = true;
            }
        }
        return $rules;
    }
}

Basically in \Magento\Catalog\Ui\DataProvider\CatalogEavValidationRules, the method called mapRules only matches the frontend class against limited number of validation rules. To apply more validation rules we need to append rules using plugin.

For server side validation, Please refer to Matthéo Geoffray answer.

I'm not sure it could be possible from install script. But I'm sure it's possible if you will create "before listener plugin" with function beforeSave() and check value there.