Add div to the_content

Hi all,

I want to add a div to the start on the_content in WordPress but I’m not sure how to write this up:

function custom_fields($content){
    return preg_replace('<div class="offer_intro"><?= get_post_meta($post->ID, "offer_intro", true); ?></div>', $content);
}
add_filter('the_content', 'custom_fields');

Can anyone help with this?
Thanks