What are the best characters to use for placeholders in file names (on Windows) AND URLs?

Question

I am writing an application in C# that will need to find placeholders in URLs and/or file names, and substitute in a value, much like this: C:\files\file{number} => C:\files\file1 Unfortunately for that example, curly braces are allowed in file names and URLs.

Can anyone please suggest some characters that I can use to denote placeholders in files and URLs? Thank you!