[_ck_]

There’s is still a problem with htmlentities, you either call your filter too soon or too late after bbPress does it’s autop, etc.

demo of bug:

WordPress & bbPress

(the and symbol is converted into an htmlentitiy, then your plugin sees the # and makes it a link) Broken all over bbpress.org

You could make sure the pattern doesn’t end with a semicolon or maybe set priority 9.