[PHP] Problems with patterns

[Phpu]

Hi,

i have a script that searches a database after a specific word.

this function check the entered word
 
function valid_word($word) {

  if(ereg("^[a-zA-Z0-9_]{3,15}$", $word)) {

   return true;

  } else {

   return false;

  }

 }

If i search for a word is ok but when i searches for a phrase it returns no result.

What do i have to modify the function?

Thank you for your reply