Why it works is due to a quirk in the handling of python strings: if an apparent escape sequence doesn't "mean anything", it is retained verbatim, including the '\' character.  This is documented in http://docs.python.org/reference/lexical_analysis.html#string-literals:

"Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, i.e., the backslash is left in the string. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken.)"

It is *very* unwise to depend on this behavior for anything except debugging, therefore those examples which do are, in my opinion, wrong.