Custom rewrite rules in Wordpress

I'm in trouble with the internal wordpress rewrite rules. I've read this thread but I still can't get any results: wp_rewrite in a WordPress Plugin

I explain my situation:

1) I have a page_template called 'myplugin_template.php' associated to a wordpress page called "mypage".

<?php
get_header();
switch ($_GET['action']) {
  case = "show" {
  echo $_GET['say'];
  }
}
get_footer();
?>

2) I need to create a rewrite rule for this link:

http://myblog/index.php?pagename=mypage&action=show&say=hello_world

If I use this url all the things works without problems but I'd like to achieve this result:

http://myblog/mypage/say/hello_world/

I really don't want to hack my .htaccess file but I don't know how I can do this with the internal wordpress rewriter.

You'll need to add your own rewrite rule and query vars - pop this in functions.php;

function my_rewrite_rules($rules)
{
    global $wp_rewrite;

    // the slug of the page to handle these rules
    $my_page = 'mypage';

    // the key is a regular expression
    // the value maps matches into a query string
    $my_rule = array(
        'mypage/(.+)/(.+)/?' => 'index.php?pagename=' . $my_page . '&my_action=$matches[1]&my_show=$matches[2]'
    );

    return array_merge($my_rule, $rules);
}
add_filter('page_rewrite_rules', 'my_rewrite_rules');


function my_query_vars($vars)
{
    // these values should match those in the rewrite rule query string above
    // I recommend using something more unique than 'action' and 'show', as you
    // could collide with other plugins or WordPress core
    $my_vars = array(
        'my_action',
        'my_show'
    );

    return array_merge($my_vars, $vars);
}
add_filter('query_vars', 'my_query_vars');

Now in your page template, replace $_GET[$var] with get_query_var($var) like so;

<?php
get_header();
switch (get_query_var('my_action')) {
    case = "show" {
        echo esc_html(get_query_var('my_say')); // escape!
    }
}
get_footer();
?>