在 Hacker News Daily 上看到這個奇怪的知識：「What does the ??!??! operator do in C? (stackoverflow.com)」，原文在 Stack Overflow 上：「What does the ??!??! operator do in C?」。
這是 trigraph，在 C89 就有了，從 Rationale for International Standard—Programming Languages—C 這邊的 5.2.1.1 可以看到 trigraph 的歷史原因：
Trigraph sequences were introduced in C89 as alternate spellings of some characters to allow the implementation of C in character sets which do not provide a sufficient number of non-alphabetic graphics
而且是強制要求實做：
Implementations are required to support these alternate spellings, even if the character set in use is ASCII, in order to allow transportation of code from systems which must use the trigraphs. AMD1 also added digraphs (see §6.4.6 and §MSE.4).
其中遇到的問題就是當年得決定 C 可以用的 charset，得考慮到很多不同機器 charset 相容性的問題：
The C89 Committee faced a serious problem in trying to define a character set for C. Not all of the character sets in general use have the right number of characters, nor do they support the graphical symbols that C users expect to see. For instance, many character sets for languages other than English resemble ASCII except that codes used for graphic characters in ASCII are instead used for alphabetic characters or diacritical marks. C relies upon a richer set of graphic characters than most other programming languages, so the representation of programs in character sets other than ASCII is a greater problem than for most other programming languages.
然後就使用了 ISO/IEC 646 這個標準 (要記得 Unicode 1.0.0 是 1991 年才出現)：
The solution is an internationally agreed-upon repertoire in terms of which an international representation of C can be defined. ISO has defined such a standard, ISO/IEC 646, which describes an invariant subset of ASCII.
The characters in the ASCII repertoire used by C and absent from the ISO/IEC 646 invariant repertoire are:
[ ] { } \ | ~ ^
後面就是定義 ?? 當作 escape digraph。
算是一個歷史產物，現在不太需要用到了...