The ANSI C standard has defined some new backslash escape sequences:
\a
for “alert.” When printed, this sequence should ring the terminal's bell.
\v
for vertical tab (this escape was already supported by many compilers).
\x
for hexadecimal constant, much like a blackslash followed by a digit introduces an octal constant.
The number of digits in an octal constant has been formally limited to three; some compilers previously allowed more. This means that \0123 is now always a two-character string: the character with octal value 012 followed by the character 3.
The digits 8 and 9 are no longer allowed in octal constants. This shouldn't be any great surprise. However, some compilers allowed \128 and took it to mean \130.