Raw string substitution problem

[Alan G Isaac]

On 12/17/2009 7:59 PM, Rhodri James wrote:
> "re.compile('a\\nc')" passes a sequence of four characters to
> re.compile: 'a', '\', 'n' and 'c'.  re.compile() then does it's own
> interpretation: 'a' passes through as is, '\' flags an escape which
> combined with 'n' produces the newline character (0x0a), and 'c' passes
> through as is.


I got that from MRAB's posts. (Thanks.)
What I'm not getting is why the replacement string
gets this particular interpretation.  What is the payoff?
(Contrast e.g. Vim's substitution syntax.)

Thanks,
Alan