GREP is a very powerful tool, but I think completely unnecessary in this case. If you are inserting a non-joiner or other special character before your first full stop you could use ordinary nested styles to do the same thing more efficiently.
Apply None up to 1 full stop, apply your character style that adds the condition up to 1 <tab or right indent tab, whichever you are using as the number separator>, and another style to add a dotted underline through the separator, if you want to add the leader that way.