RUF055 ignores backslashes in the repl argument of re.sub

[dscorbett]

In Ruff 0.8.1, unnecessary-regular-expression (RUF055) reports a false positive for re.sub when the replacement string contains a backslash.

$ cat ruf055.py
import re
print(re.sub(" ", r"\n", "a b"))

$ python ruf055.py
a
b

$ ruff check --isolated --preview --select RUF055 ruf055.py --fix
Found 1 error (1 fixed, 0 remaining).

$ cat ruf055.py
import re
print("a b".replace(" ", "\\n"))

$ python ruf055.py
a\nb

This issue was mentioned in #14679 (comment) with the example of \g<0>, which was deemed acceptably implausible. Because a replacement string can also include character escapes, this issue is plausible.

Alternatively, this could be considered a true positive, in which case the fix needs to interpret escape sequences in the replacement string.

[MichaReiser]

Thanks.

Okay, so the part that we overlooked from the specification is:

if it is a string, any backslash escapes in it are processed. That is, \n is converted to a single newline character, \r is converted to a carriage return, and so forth.

[MichaReiser]

@AlexWaygood pointed out that this could be addressed by #14679

[AlexWaygood]

Thanks @dscorbett, as ever! You were quite right that #14679 would not have fixed this in its earlier form. I pushed some updates to it, so I think it now handles everything correctly, though. Let us know if you see any other issues with the check!