Got error 'repetition-operator operand invalid' from regexp

select count(gc_ads.n) from gc_ads,gc_index where ( (t1<=1713491897 OR t1=0) AND (t2>=1713491897 OR t2=0) AND status = 'enabled' AND x_paypal_disabled = '0' ) and ((gc_index.all_text REGEXP '*sex')) AND gc_ads.n = gc_index.n