Your code lacks a lot of basic principles.
A sentence in html could end like this.<BR>
or like this.</p>
Or generally, like this. (<--space character)
Or perhaps a tab.
So you need to cover all of those AFTER the punctuation so you don't breakup image.jpg or otherpage.htm or ../page.htm, etc. (hey there's another example -- etc!!!)
So EFFECTIVELY breaking up sentences using preg is MUCH more complex than you're getting at, but I hope I've opened your understanding to the scope.
I can't offer much direct help, there are some great articles out there, but if you want a great tester, go to my site at:
http://samuelfullman.com/team/php/tools/regular_expression_tester_p.php
Build you expression in the top text area, and refer to a page (anywhere on the web) that has sentences. If you get it right, you'll get a list (array) of all your sentences numbered below.
Sam Fullman
Compass Point Media
and you can rapidy build your expression and test it out. this can be a humbling experience.