src/Blocks/BlockHorizontalRule.php
public function validateLineType($lineNumber)
    {
        $dataObject = $this->factory->getDataObject();
        $lineContent = $dataObject->getLine($lineNumber);

        $contentTrimmed = str_replace(['*', '-', '_'], '', $lineContent);
        if (!empty(trim($contentTrimmed))) {
            return false;
        }

        return (bool) preg_match('/^([\*|\*\s|\-|\-\s|\_|\_\s]{3,})/', $lineContent);
    }
public function validateLineType($lineNumber)
    {
        $dataObject = $this->factory->getDataObject();
        $lineContent = $dataObject->getLine($lineNumber);

        $contentTrimmed = str_replace(['*', '-', '_'], '', $lineContent);
        if (!empty(trim($contentTrimmed))) {
            return false;
        }

        return (bool) preg_match('/^([\*|\*\s|\-|\-\s|\_|\_\s]{3,})/', $lineContent);
    }
@param int $lineNumber

@return bool