Link post images to post

Yes, don’t use regex for parsing HTML but use DOM instead.

This is how I would work it out :

In your toolbox, a function to get innerDOM of a node :

// GET INNER HTML OF A NODE
function DOMinnerHTML($element)  
{ 
    $innerHTML = ""; 
    $children = $element->childNodes; 
    foreach ($children as $child) 
    { 
        $tmp_dom = new DOMDocument(); 
        $tmp_dom->appendChild($tmp_dom->importNode($child, true)); 
        $innerHTML.=trim($tmp_dom->saveHTML()); 
    } 
    return $innerHTML; 
} 

Then a filter to apply a custom function :

function change_destination_links_for_images( $content ) {   
    $dom = new DOMDocument();
    // THIS IS HACK TO LOAD STRING WITH CORRECT ENCODING
    // JUST OUTPUT <--?xml encoding="UTF-8"--> IN HTML SO NO HARM
    $dom->loadHTML( '<?xml encoding="UTF-8">' .  $content );

    // GET ALL <a> NODE
    foreach ( $dom->getElementsByTagName('a') as $node ) {
        // GET HREF 
        $link_href = $node->getAttribute( 'href' );
        // USE INNER OF THIS <a> NODE AS NEW DOC TO EXTRACT IMG
        $dom_node = new DOMDocument();
        $inner = DOMinnerHTML($node);
        $dom_node->loadHTML($inner);
        // CHECK IF IMAGE INSIDE THE LINK
        if ( $dom_node->getElementsByTagName('img') {
           // IF SO REPLACE HREF
           $node->setAttribute('href', get_permalink() );
           // RETURN MODIFIED DOM
           $content = $dom->saveHTML();
        }
    }
// RETURN CONTENT
return $content;
}
// APPLY FILTER
add_filter( 'the_content', 'change_destination_links_for_images' );

This is a quick adaptation of a working job I have done for another purpose so it may have errors in it. Hope this helps.