How to convert characters or number to underscore and after that conca

I work on SQL server 2012 i face issue I need to convert characters and numbers to underscore "_"

and after that concatenate it based on length of number or characters

so if one character or one number will be _

so if two character or two number will be __

so if three character or three number will be ___

etc

so How to make select query do above on test sample below

create table portionstable(PortionKey nvarchar(20))

insert into portionstable(PortionKey)

values

('123'),

('abc'),

('a'),

('ab'),

('12')

select * from portionstable___________what I try but not workingselect concat(replace(PortionKey,PortionKey,'_' )) as portionkeyfrom portionstable

so how to do it