[PHP] Newbie pattern question

[Phpu]

Hi,
I have this function

 function validate_name($name) {

  if(ereg("^[a-zA-Z0-9_]{2,30}$", $name)) {

   return true;

  } else {

   return false;

  }

 }

If i enter a name like John for exemple everything is ok but if i enter John Doe the 
function return false.

Where is the problem?

Thanks