[hvdijk]

That's a bug, there is no such thing as an invalid trigraph. ?? followed by any character other than =, /, ', (, ), !, <, >, or - is not a valid trigraph, but that doesn't make it an invalid trigraph, that just makes it not a trigraph, it's perfectly valid to have ??? in a comment, or in a string literal.

[DonHopkins]

Are you telling me that C compilers in the early 90's had bugs and confusing error messages??!!?!??? WTF?!??!?!?

[benj111]

Oh so that's why their called trigraphs, because there's 3 valid states?

Valid Invalid ??? (Exercise for the reader to decide if this is a trigraph or not)