Episode Four: Not Enough Keys in the Keyboard

The basic character set of the C programming language is a subset of the ASCII character set that includes nine characters which lie outside the ISO 646 invariant character set. Trigraphs were invented as a way of entering source code using keyboards that support any version of the ISO 646 character set.

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

Handling of trigraphs is done at the preprocessing stage. This means they are replaced everywhere, which can became the cause of subtle bugs.

· Within a string literal:

"He said 'Hello???'." becames "He said 'Hello?^.".

· Within a line comment:

// Whats wrong with this??/
void i = 0; // This line is also inside the line comment.

Digraphs were supplied as more readable alternatives to six of the trigraphs.

Digraph	Replacement
<:	[
:>	]
<%	{
%>	}
%:	#
%:%:	##

Unlike trigraphs, digraphs are handled during tokenization, so they pose no harm.