So sounds to ME like you're looking for S'es but which must begin with :::.
Seems to me you might just have to use two operations, I don't know of any way to convey that english equivalent using regexp (you can always find a string in regex but you may not be able to 'express' a string :-)
/:::.s/i
you could also use if(regexp(),1,0) AS fieldName, and then sort or do something based on the value of that expression.
Sam