I'm surprised it gets to "\x00".
String s = "\\x01\\x03\\x04\\x01\\x00\\xC1\\x94";
It's really better to avoid the String class, by the way, and use C strings, (char arrays).
The String class is resource heavy and can lead to memory fragmentation.
A backslash in C++ is used to signify an 'escape sequence', like '\n', '\t' etc.
To include an actual backslash in a string, it must itself be an escape sequence.
More info here, including a chart of escape sequences:-
Here's a better chart, (non-Microsoft-specific):-