Use other character than percent for formats?

[fluks]

The percent character is used for formats. It is often seen in URLs and there might be bugs relating to the substitution. Those possible bugs should of course be fixed, but maybe it would be better to use something else, preferably a character that can never be in a URL. I'm not sure after research what such a character could be, use a dollar sign?

This will of course break many formats, maybe those could be converted automatically on update.