The UniqueID of a control is delemited with the '$', is it possble that the delimiter can change, and if so, is there a property somewhere that contains the delimiting character?
Given that there are the protected properties IdSeparator and ClientIDSeparator, I would regard it as subject to change and avoid writing code that make assumptions that a certain character is used as separator. Both those properties are of the type Char, so I would find it unlikely that they would change into using multiple-character separators, but that is just an guess, and also something that I would avoid relying on.