An example would be a site that has thumnails in the url:
And the actual picture is in the URL:
(note the path is dependant on the first letter of the pic name).
I don't see anyway I can do this with the existing URL substitutions with 3 parameters.
The problem is that the letter after f varies too - so I cannot see anyway to do it with the current url substiitution parameters.
and so on.