Url Rewriting a dynamic wordpress page

thanks for you reply,

So after much digging, it’s working :

function myplugin_rewrite_tag_rule() {
   add_rewrite_tag( '%symbol%', '([a-zA-Z0-9_]+)' );
   add_rewrite_rule( '^crypto-monnaies/([a-zA-Z0-9_]+)/?$', 'index.php?
pagename=showcoin&symbol=$matches[1]','top' );
}
add_action('init', 'myplugin_rewrite_tag_rule');

function add_query_vars( $query_vars ) {
   $query_vars[] = 'symbol';
   return $query_vars;
}
add_filter( 'query_vars', 'add_query_vars' ); 

Please note that if you dynamic page is a second level page, you have to call the ?pagename=parent-page/child-page (with the slash).

To resolve my problem i put the “showcoin” page to first level.