how to define Grails url mappings with optional parameters not at end of url

Question

I can only find examples of Grails url mappings with optional parameters at the end of the url; however, I need to define optional parameters followed by a required suffix which is not a file extension.

e.g. /a?/b?/c which would match all of the following:

1. /c
2. /a/c
3. /a/b/c

However, if I just define /a?/b?/c, then 'c' is matched as 'b' in #2 above.

How can a url mapping be defined, so that c is always matched first?