2 Lexical conventions [lex]

2.4 Trigraph sequences [lex.trigraph]

1

Before any other processing takes place, each occurrence of one of the following sequences of three characters (“trigraph sequences”) is replaced by the single character indicated in Table [tab:trigraph.sequences].

Table 1 — Trigraph sequences
Trigraph Replacement Trigraph Replacement Trigraph Replacement
??= # ??( [ ??< {
??/ \ ??) ] ??> }
??' ^ ??! | ??- ~
2

[ Example: 

??=define arraycheck(a,b) a??(b??) ??!??! b??(a??)

becomes

#define arraycheck(a,b) a[b] || b[a]

 — end example ]

3

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