trigraphs and digraphs

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



Digraph     Equivalent
=======     ==========
  <:             [
  :>             ]
  <%             {
  %>             }
  %:             #
  %:%:           ##

References
wikipedia
CERT C Secure Coding Standard