The point about a performance penalty is fair---my PR does add a search for the '@' (which I spelled as '`' in my example above) sigil whenever it encounters a '|'. (Though I'm not sure how big the impact would be? Format strings are small so a quick scan through it should be pretty fast. Could be measured.)

I am missing how my PR changes the current behavior, though? As far as I can tell it should behave exactly the same unless there is a '@' in the format string.