Fixed error when focus keyword contains a /

Fixes support requests:
* http://wordpress.org/support/topic/warning-preg_match-in-class-metaboxphp-on-line-1690
* http://wordpress.org/support/topic/preg_match-functionpreg-match-unknown-modifier-n-pluginswordpress-seoa
* http://wordpress.org/support/topic/plugin-cause-a-huge-error
* http://wordpress.org/support/topic/php-error-46
* http://wordpress.org/support/topic/warning-while-while-publishing-new-post
* http://wordpress.org/support/topic/cant-publish-or-save-draft-new-posts

Replaced the regular expression delimiters by ` which is a much less likely character to be in a focus keyword