When does the escape character work within raw strings?

[walterbyrd]

I know that

s = r'x\nx'

means 'x' followed by a literal '\' followed by an 'n' (the '\n' is
not a carriage return).

s = r'x\tx'

means 'x' followed by a literal '\' followed by an 't' (the '\t' is
not a tab).

But, boundries seem to work differently.

s = re.sub(r'\bxxx\b', 'yyy', s)

Is *not* going to look for a literal '\' followed by 'b'

So I am confused about when escapes work within raw strings, and when
do escapes not work within raw strings.