Remove all the old tags in post using code.

Hi, I been using one plugin that assigns tags to posts containing specific text strings comparing with external file when I press de PUBLISH button. When I make changes from an old post and push UPDATE the new text append new tags to the post. And the old ones still on it. I need to know how modify this code to make posible the erase of old tags before to put the new ones. I read all information in codex.wordpress.org and nothing help me. I looking on the web and only get this http://wordpress.stackexchange.com/questions/49248/remove-tags-from-posts-in-php and this url http://stackoverflow.com/questions/3356380/wordpress-delete-all-tags-by-post-id but when I use this codes nothing happend or just get an error massage.

• add_action( 'save_post', 'jpl_docs_autotag' );
  function jpl_docs_autotag($post_id) {
      $post = get_post($post_id);
      if ( intval($post->ID) > 0 ) {
  
  // HERE IS WHERE I PUT THE CODE THAT I´VE TRIED
  
  		$options = get_option( 'docs_autotags' );
  		if ( count($options) > 0 ) {
  			foreach ( $options as $pattern => $tags ) {
  				if ( strpos($post->post_content, $pattern) )
  					wp_set_post_tags( $post->ID, $tags, TRUE ); // TRUE means append
  			}
  		}
      }
  }

Thanks