completion for printf-style format strings

[Felix Rosencrantz <f.rosencrantz@xxxxxxxxx>]

Does anyone have code to perform completion of printf-style format strings?

I think it was suggested that it might be possible to do with 
_regex_arguments. but not sure if there is an example somewhere or a
utility function.

I'm not sure if such completion would help reduce typing, but being
able to get a context based listing of possible values could help skip
a trip to the documentation..

-FR.