Which characters are allowed (by default) in a SKU?

Question

A simple question with no hits on Google.

What are the characters that are allowed in a SKU? The entire UTF-8 character set?

Answer 1

Diving into the core code... The validation starts at Mage_Catalog_Model_Product_Attribute_Backend_Sku and simply checks the length to make sure it's less than 64 characters long...

/**
 * Validate SKU
 *
 * @param Mage_Catalog_Model_Product $object
 * @throws Mage_Core_Exception
 * @return bool
 */
public function validate($object)
{
    $helper = Mage::helper('core/string');

    if ($helper->strlen($object->getSku()) > self::SKU_MAX_LENGTH) {
        Mage::throwException(
            Mage::helper('catalog')->__('SKU length should be %s characters maximum.', self::SKU_MAX_LENGTH)
        );
    }
    return parent::validate($object);
}

Then proceeds to call through to its parent's method... That only checks for uniqueness and requiredness of the attribute.

/**
 * Validate object
 *
 * @param Varien_Object $object
 * @throws Mage_Eav_Exception
 * @return boolean
 */
public function validate($object)
{
    $attrCode = $this->getAttribute()->getAttributeCode();
    $value = $object->getData($attrCode);
    if ($this->getAttribute()->getIsRequired() && $this->getAttribute()->isValueEmpty($value)) {
        return false;
    }

    if ($this->getAttribute()->getIsUnique()
        && !$this->getAttribute()->getIsRequired()
        && ($value == '' || $this->getAttribute()->isValueEmpty($value)))
    {
        return true;
    }

    if ($this->getAttribute()->getIsUnique()) {
        if (!$this->getAttribute()->getEntity()->checkAttributeUniqueValue($this->getAttribute(), $object)) {
            $label = $this->getAttribute()->getFrontend()->getLabel();
            throw Mage::exception('Mage_Eav',
                Mage::helper('eav')->__('The value of attribute "%s" must be unique', $label)
            );
        }
    }

    return true;
}

So apparently, to the Magento Core team, anything goes for a SKU so long as it's under 64 characters, non-null, and unique. I suppose the underlying database type is also a consideration as well.