> gettext.c2py("n()")(lambda: os.system("sh"))
> gettext.c2py("1()")(0)

Empty parentheses should be disallowed. Function calls are not allowed in plural expression. And non-integer argument should be disallowed either, just as Serhiy's example shows.

> What if make repeated replacements with regular expression r'([^?:]*?)\?([^?:]*?):([^?:]*)'?

How does it work for '1?2:3?4:5'? :-( I am considering a parser.