If I have a description like:
\"We prefer questions that can be answered, not just discussed. Provide details. Write clearly and simply.\"
I'd probably use any of the multitudes of substring/string-split functions in PHP (some mentioned here already). But also look for ". " OR ".\n" (and possibly ".\n\r") instead of just ".". Just in case for whatever reason, the sentence contains a period that isn't followed by a space. I think it will harden the likelihood of you getting genuine results.
Example, searching for just "." on:
"I like stackoverflow.com."
Will get you:
"I like stackoverflow."
When really, I'm sure you'd prefer:
"I like stackoverflow.com."
And once you have that basic search, you'll probably come across one or two occasions where it may miss something. Tune as you run with it!