How can I edit post data before it is saved?

I have a plugin and I would like to be able to run the post content through some filters before it is saved to the database. From looking at the plugin api, I see that two hooks that look like they might be helpful:

save_post
wp_insert_post

The only problem is that it looks like save_post does not need return a variable, and so I don’t know how to filter the content, and wp_insert_post looks documented.

I’d like to do something like this:

add_action('whatever_hook_name','my_function');

function my_function($post_content){
    return $post_content.' <br> This post was saved on '.time();
}

I am going to do something more useful than append a timestamp, namely running some regex filters, but this is the general type of filter / action I’m trying to add.

Update

Please note that I want to intercept the data on it’s way to being saved in the database – not when it is being displayed in the post (eg: Not by adding a filter to the_content)

Solutions Collecting From Web of "How can I edit post data before it is saved?"

The wp_insert_post_data filter can do that:

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

function filter_post_data( $data , $postarr ) {
    // Change post title
    $data['post_title'] .= '_suffix';
    return $data;
}

Use filter content_save_pre exactly like the_content — the difference is that it operates when a post is saved, rather than displayed.

http://codex.wordpress.org/Plugin_API/Filter_Reference/content_save_pre

If you just want to add something similar at the end of all the posts, then I would suggest you use the the_content filter.

function append_to_content( $content ) {
    global $post;
    return $content.'<br />This post was saved on '.$post->post_date;
}
add_filter( 'the_content', 'append_to_content' );

add the following code to active theme.

Here I am replacing with [shell] before saving.

add_filter( 'content_save_pre' , 'my_sanitize_content' , 10, 1);
function my_sanitize_content( $value ) {
          return str_replace("<shell>", "[shell]", $value );
}