Wrap h1-h6 in a div

Try this code

function wrap_heading_with_div( $content ) {
    $heading = '/<h\d.*?>(.*?)<\/h\d>/ims';
    $wrapper="<div class="title">$0</div>";
    $content = preg_replace($heading, $wrapper, $content);
    return $content;
}
add_filter( 'the_content', 'wrap_heading_with_div' );

Live Demo