Newbie needs help with ereg regular expression

Hi,

I have to repost this to restate the question. No disrespect to the original
reply.

Please can someone help regarding providing a regular expression for entering
a set of characters with letters, numbers, periods (.), hyphens (-), or
underscores (_) only.

I need to do this using ereg

I've got as far as this but it doesn't quite work:

$pattern="^([a-z0-9.-_]{0,30})$";
if(ereg($pattern,$_SESSION['id']))
....


Thank you in advance.

Kind regards,

Raj (newbie)