I started getting this message, this afternoon, all of a sudden. I wasn't getting it yesterday, and I haven't gotten it most of the day today.
Warning: preg_match_all() [function.preg-match-all]: Unknown modifier 'P' in /home/bamajr.com/public_html/wp-content/plugins/wordpress-seo/admin/linkdex/linkdex.php on line 476
This error appears on the "New Post" page and has something to do with the "Focus Keyword" section.
When I put "Stop SOPA/PIPA: The Great Internet Blackout" in the "Focus Keyword" section, this error occurs. It must have something to do with one of the two special characters.
I already rated your plugin 5 stars, as I've been using it for a while with great success and I understand this isn't a traditional use of the "Focus Keyword" section.