Piwik\Plugin\Manager::isValidPluginName PHP Method

    public function isValidPluginName($pluginName)
    {
        return (bool) preg_match('/^[a-zA-Z]([a-zA-Z0-9_]*)$/D', $pluginName);
    }

 private function getPluginNameIfNonceValid($nonceName)
 {
     $nonce = Common::getRequestVar('nonce', null, 'string');
     if (!Nonce::verifyNonce($nonceName, $nonce)) {
         throw new \Exception(Piwik::translate('General_ExceptionNonceMismatch'));
     }
     Nonce::discardNonce($nonceName);
     $pluginName = Common::getRequestVar('pluginName', null, 'string');
     if (!$this->pluginManager->isValidPluginName($pluginName)) {
         throw new Exception('Invalid plugin name');
     }
     return $pluginName;
 }