Wordpress - Removing any and all inline styles from the_content()

If we want to remove all inline styles, then just simply need to add the following code in functions.php.

add_filter('the_content', function( $content ){
    //--Remove all inline styles--
    $content = preg_replace('/ style=("|\')(.*?)("|\')/','',$content);
    return $content;
}, 20);

Just add this to your functions.php.

Note: This filter works at the time of saving/updating the post.

add_filter( 'wp_insert_post_data' , 'filter_post_data' , '99', 2 );

function filter_post_data( $data , $postarr ) {

    $content = $data['post_content'];

    $content = preg_replace('#<p.*?>(.*?)</p>#i', '<p>\1</p>', $content);
    $content = preg_replace('#<span.*?>(.*?)</span>#i', '<span>\1</span>', $content);
    $content = preg_replace('#<ol.*?>(.*?)</ol>#i', '<ol>\1</ol>', $content);
    $content = preg_replace('#<ul.*?>(.*?)</ul>#i', '<ul>\1</ul>', $content);
    $content = preg_replace('#<li.*?>(.*?)</li>#i', '<li>\1</li>', $content);

    $data['post_content'] = $content;

    return $data;
}

---

Note: This filter works at the time when function the_content() is executed.

add_filter( 'the_content', 'the_content_filter', 20 );

function the_content_filter( $content ) {
    $content = preg_replace('#<p.*?>(.*?)</p>#i', '<p>\1</p>', $content);
    $content = preg_replace('#<span.*?>(.*?)</span>#i', '<span>\1</span>', $content);
    $content = preg_replace('#<ol.*?>(.*?)</ol>#i', '<ol>\1</ol>', $content);
    $content = preg_replace('#<ul.*?>(.*?)</ul>#i', '<ul>\1</ul>', $content);
    $content = preg_replace('#<li.*?>(.*?)</li>#i', '<li>\1</li>', $content);
    return $content;
}

I tried the method above with the saving/updating but didn't worked for me so I went from another approach. I exported the whole wp_posts table, opened it in Sublime and did a regex replace. I used style="*.*?" to find all cases and replaced them with emptyness. Then droped the old table's content and imported the new one.

If any one try this method - please make sure you have a clear back up in case there are some other post types in the wp_post table and the things got bit messy.