I found a way that works for me. It is not 100% exact but it eliminates all strings that contain more than just the word I am looking for because I check for the string not containing individual spaces too. By the way you don't need these " ". jQuery knows you are looking for a string. Make sure you only have one space in the :contains( ) part otherwise it won't work.
<p>hello</p>
<p>hello world</p>
$('p:contains(hello):not(:contains( ))').css('font-weight', 'bold');
And yes I know it won't work if you have stuff like <p>helloworld</p>