Got error 'empty (sub)expression' from regexp

SELECT c.* FROM global_content c, components_installed i WHERE c.componentID = i.componentID AND i.componentStatus AND ((placementDistribution = 'all' OR (placementSectionID REGEXP '' AND placementDistribution = 'selected') OR (placementSectionID NOT REGEXP '' AND placementDistribution = 'except'))) AND placementStatus = 1 ORDER BY placementSequence