What characters to use in a dunin id

What are the characters that should be used in a dunin-id in order to make it a valid url and keep it nice to understand and to read?

Brainstorming

Dunin ID should be a valid url that allows this characters. ?, &and =can be reserved for queries What character to use to separate the dunin-agent-locator from the rest of the dunin ID? Originally Idea: Three slashes /// but: Dunin ID becomes long because three characters Hard to read when stacked-dunin-agent-locator An other Idea: ~ but: Special meaning in some file systems Some user agents encode it Best solution: _ For separating words is still -left