preg_match: number-alphabets and commas only

How do I write a regular expression which matches number-alphabets and commas only?

I came out with this one below but it doesnt work - it accepts other punctuation marks as well!

# check for matches number-alphabets and commas only
  if(!preg_match('/([a-zA-Z0-9]|[a-zA-Z0-9\,])/', $cst_value))
  {
   $error = true;
   echo '<error elementid="usr_username" message="'.$cst_name.' - please use number-alphabets and commas only."/>';
  }

Many thanks, Lau