Regarding C programming language, the descriptions of escape sequence does not resolve the output of following line in visual studio 2008.
char * str = "??/abc";
printf( "%s", str );
it prints only "bc".
Checking the memory pointed by str, we can find that "??/a" is working as "\a"
Searching on google we can't find anything related to "??/" being treated as backslash.
To expand on AProgrammer's comment, C defines a set of trigraphs for people whose keyboards are missing certain characters. These begin with '??', followed by another character. In particular, '??/' is the trigraph for '\'.
This translation is done very early on in the complilation process, so the compiler just sees '\a'.
See MSDN