#WordPress template part that extracts the heading anchors using #regex in order to build an index to navigate through the content

indexed-content.php

<?php
$content = get_the_content();
$matches = array();
preg_match_all('/\<(h[123456]).*?\sid+\=\"([0-9a-zA-Z\-]*?)\"\s?.*?\>(.+?)\<\/\1\>/', $content, $matches, PREG_SET_ORDER);
$links = array();

foreach($matches as $match){
    $links[] = array(
        'id' => $match[2],
        'content' => $match[3]
    );
}; ?>

<div class="indexed-section">
    <nav class="indexed-section__index">
        <?php foreach($links as $link): ?>
        <a href="#<?= $link['id'] ?>" class="indexed-section__link"><?= $link['content'] ?></a>
        <?php endforeach; ?>
    </nav>
    <main class="indexed-section__content">
        <?php the_content(); ?>
    </main>
</div>