C语言中的Trigraph sequences

以下对Trigraph sequences的描述摘自ANSI C标准，一般很少提到和用到。

2.2.1.1 Trigraph sequences

   All occurrences in a source file of the following sequences of
three characters (called trigraph sequences /5/)are replaced with the
corresponding single character.

         ??=      #
         ??(      [
         ??/      \
         ??)      ]
         ??'      ^
         ??<      {
         ??!      |
         ??>      }
         ??-      ~

No other trigraph sequences exist.  Each ? that does not begin one of
the trigraphs listed above is not changed.

Example

   The following source line

         printf("Eh???/n");

becomes (after replacement of the trigraph sequence ??/ )

         printf("Eh?\n");