higle,
using s?printf in this context is not a kludge at all! It's simply the most straightforward way to get the job done, we're functions meant for this purpose.

Without thinking too much about it, I'd guess that a substitution regexp would require the e modifier to get the formatting job done. So it would be more cumbersome, more computationally expensive, harder to debug, probably uglier too :-)

-- TMTOWTDI