reaction: fix trigger regex to work with non-alphanumeric start/end.

reaction: fix trigger regex to work with non-alphanumeric start/end.

only match on word boundaries at an end of a trigger if the character at
that end of the trigger is a word character.  In other words, the
trigger "test" should require word boundaries on each side, but the
trigger "@test" should only require one on the right side.