private function readDigits()
    {
        [$char, $code] = $this->readChar();

        if ($code >= 48 && $code <= 57) { // 0 - 9
            $value = '';

            do {
                $value        .= $char;
                [$char, $code] = $this->moveStringCursor(1, 1)->readChar();
            } while ($code >= 48 && $code <= 57); // 0 - 9

            return $value;
        }

        if ($this->position > $this->source->length - 1) {
            $code = null;
        }

        throw new SyntaxError(
            $this->source,
            $this->position,
            'Invalid number, expected digit but got: ' . Utils::printCharCode($code)
        );
    }