You can use any of the expressions ** or []{2} and use # as the replace string. It will not work for occurrances where you have more than two asterisks because replace does work recursively. So you can use replace in a REPEAT loop. After making a call to replace you can have a BRANCH step which looks for the regeular expression []{2} or ** and if it is not found exit out of the loop, otherwise continue with the next iteration.
Ok, thank you Rupinder.