User::validateValueSingle

static function validateValueSingle($value, FieldType $fieldType)
{
	$value = static::toSingleValue($fieldType, $value);

	$value = trim($value);

	$isUser = (mb_strpos($value, 'user_') !== false);
	if ($isUser)
	{
		return $value;
	}

	$isGroup = (mb_strpos($value, 'group_') !== false);
	if ($isGroup)
	{
		return $value;
	}

	$isNumber = preg_match('#^[0-9]+$#', $value);
	if ($isNumber)
	{
		return $value;
	}

	$isDocumentGroup = AutomationHelper::isDocumentUserGroup($value, $fieldType->getDocumentType());
	if ($isDocumentGroup)
	{
		return $value;
	}

	return null;
}