Adding ID to headings in WordPress

id-heading.php

<?php

/**
 * Adds "id" attribute to heading tags in WordPress post content using the "the_content" filter and regex.
 */
function anchor_headings( $content ) {

    $content = preg_replace_callback( '/(\<h[2-3](.*?))\>(.*)(<\/h[2-3]>)/i', function( $matches ) {
        if ( ! stripos( $matches[0], 'id=' ) ){
            $slug = sanitize_title( $matches[3] );
            $matches[0] = $matches[1] . $matches[2] . ' id="' . $slug . '">' . $matches[3] . ' <a href="#' . $slug . '" aria-hidden="true" class="anchor">#</a>' . $matches[4];
        }
        return $matches[0];
    }, $content );

    return $content;

}

add_filter( 'the_content', 'anchor_headings' );

?>