function replace_content_on_the_fly($text){
	$replace = array(
		// 'words to find' => 'replace with this'
		'wordpress' => '<a href="http://www.wordpress.org/">wordpress</a>',
		'google' => '<a href="http://www.google.com/">excerpt</a>',
		'function' => '<a href="#">function</a>'
	);
	$text = str_replace(array_keys($replace), $replace, $text);
	return $text;
}
add_filter('the_content', 'replace_content_on_the_fly');
add_filter('the_excerpt', 'replace_content_on_the_fly');