You haven't encoded the HTML tags in your example results, but I can guess what is happening: all 5 of the "net"s in the text are being highlighted, whether or not they are a whole word or if they are proceeded by a dot.
The problem is using the keyword string as a regular expression. For example, a dot is a special character in a Regex expression which matches any character - not just a dot!
If you want to use Regex to do this, you will need to do much more processing of the keyword string.
There is a ( free ) program called Expresso that will help you form your expression:
http://www.ultrapico.com/Expresso.htm[
^]
Nick