preg_replace problem

Hi, I am having this problem with preg_replace. I want the following thing but I can't understand what regular expression I should put. I want to replace all occurences of a given **whole** word into a string. For example: I want "A dog jumped over a ladder" to become "<xyz>A</xyz> dog jumped over <xyz>a</xyz> ladder". I **don't** want: "A dog jumped over a ladder" to become "<xyz>A</xyz> dog jumped over <xyz>a</xyz> l<xyz>a</xyz>dder". You understand what I mean? I can't do that will str_replace, but I know it would be quite easy with preg_replace. Can someone help me? Thanks Vincent