Delete/Replace words from X to Y in post content

Untested, but I believe you can do something like this:

add_action( 'init', function() {

    // since were editing content, we want to be very very careful
    // well do a dry run output first to make sure were doing what we want
    $dryrun = true;

    // Regex pattern finds anything between the X and Y bounds.
    $pattern = '/(XXXX)(.+?)(YYYY)/gms';

    // Get and cycle through ALL published `posts`
    $all_posts = get_posts('posts_per_page=-1');
    foreach ($all_posts as $a_post) {

        // replace the pattern with our new content.
        // If the content is to be preserved, we could use $2 in the second arg
        $new_content = preg_replace($pattern, "New content here", $a_post->post_content);

        // if somthing was replaced, lets continue
        if ( $new_content != $a_post->post_content ) {

            // display what _would_ be changing
            if ( $dryrun ) {
                echo "<div>update post #{$a_post->ID} to:</div>";
                echo "<pre>{$new_content}</pre>";
            } else {
                // update the post_content with our new content
                wp_update_post(array(
                    'ID' => $a_post->ID,
                    'post_content' => $new_content,
                ));             
            }
        } else {
            // nothing was changed, if dry run, just print the notice.
            if ( $dryrun ) {
                echo "<div>nothing to update in post #{$a_post->ID}</div>";
            }
        }
    }
});

Even though this code has a dry run on it, always back up your database content and verify it before doing mass-modifying like this, things can go horribly wrong very quickly.

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