symfony/css-selector · error · ExpressionErrorException

Extension " " not registered.

Error message

Extension "%s" not registered.

What it means

ExpressionErrorException thrown by Translator::getExtension when the requested extension name is not registered on the translator instance. Extensions (Html, Function, PseudoClass, Attribute, etc.) must be explicitly added via addExtension/registerDefaultExtensions; getExtension is the internal lookup used by translator features and is also public API.

Solutions

1. Register the extension first: $translator->addExtension(new HtmlExtension()); or call registerDefaultExtensions().
2. Use Translator::fromCssConverter()/default constructors that pre-register the standard extensions.
3. Verify the exact extension name/key (case-sensitive) used with getExtension().
4. Catch ExpressionErrorException and fall back to a translator configured with defaults.

Example fix

// before
$translator = new Translator();
$ext = $translator->getExtension('html'); // throws
// after
$translator = new Translator();
$translator->registerDefaultExtensions();
$translator->addExtension(new HtmlExtension());
$ext = $translator->getExtension('html');

function hasExtension(\Symfony\Component\CssSelector\XPath\Translator $t, string $name): bool {
    try { $t->getExtension($name); return true; } catch (ExpressionErrorException $e) { return false; }
}

function extensionExists(array $extensions, string $name): bool {
    return isset($extensions[$name]);
}

try {
    $ext = $translator->getExtension('html');
} catch (ExpressionErrorException $e) {
    $translator->registerDefaultExtensions();
    $translator->addExtension(new \Symfony\Component\CssSelector\XPath\Extension\HtmlExtension());
    $ext = $translator->getExtension('html');
}

When it happens

Trigger: Calling $translator->getExtension('html') on a translator that was never configured with the Html extension, mistyping an extension name, or constructing Translator manually without registerDefaultExtensions().

Common situations: Building a Translator with new Translator() and no extensions before translating HTML-specific selectors, DI misconfiguration, typos like 'htm' or 'pseudo-class' with wrong casing.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

• Got too deeply nested
• Invalid series: " ".
• "*:nth-of-type()" is not implemented.
• Expected a single string or identifier for :contains(), got
• Expected a single string or identifier for :lang(), got

AI-assisted analysis of symfony/css-selector@08e2905152 (2026-09-14). Data as JSON: /api/errors/9087de3b39441c3e. Report an issue: GitHub.

Appendix: source

Thrown at XPath/Translator.php:135

        $this->extensions[$extension->getName()] = $extension;

        $this->nodeTranslators = array_merge($this->nodeTranslators, $extension->getNodeTranslators());
        $this->combinationTranslators = array_merge($this->combinationTranslators, $extension->getCombinationTranslators());
        $this->functionTranslators = array_merge($this->functionTranslators, $extension->getFunctionTranslators());
        $this->pseudoClassTranslators = array_merge($this->pseudoClassTranslators, $extension->getPseudoClassTranslators());
        $this->attributeMatchingTranslators = array_merge($this->attributeMatchingTranslators, $extension->getAttributeMatchingTranslators());
        $this->relativeCombinationTranslators = array_merge($this->relativeCombinationTranslators, $extension->getRelativeCombinationTranslators());

        return $this;
    }

    /**
     * @throws ExpressionErrorException
     */
    public function getExtension(string $name): Extension\ExtensionInterface
    {
        if (!isset($this->extensions[$name])) {
            throw new ExpressionErrorException(\sprintf('Extension "%s" not registered.', $name));
        }

        return $this->extensions[$name];
    }

    /**
     * @return $this
     */
    public function registerParserShortcut(ParserInterface $shortcut): static
    {
        $this->shortcutParsers[] = $shortcut;

        return $this;
    }

    /**
     * @throws ExpressionErrorException
     */

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