Erro: -420 Database Error

Got error 'empty (sub)expression' from regexp

Statement:
select ifnull(parent.alias, '') as parentalias, ifnull(parent.title, '') as parent, pages.*,
if(pages.authortabledefid=2,clients.name,users.firstname) as authorname,
clients.bio
from pages
left join pages as parent on parent.id = pages.parentid
left join clients on clients.id=pages.authorid and pages.authortabledefid=2
left join users on users.id=pages.authorid and pages.authortabledefid!=2
where pages.tags REGEXP ''