ruff W605: fix changes program behavior

Hypothesis Nemotron 3 Super predicted to fire fired 1×

When the string is used in a regex context via re.sub or similar, doubling backslashes changes the number of escape layers, potentially requiring compensatory changes elsewhere in the pattern.

Program

Output

(no stdout)
/w/prog.py:1: SyntaxWarning: invalid escape sequence '\.'

(no stdout)

Judge Nemotron 3 Ultra advisory intended-change · low

The rule rewrites an invalid escape sequence '\.' to a raw string r'\.' to eliminate the SyntaxWarning, which is the documented purpose of the fix. The program's behavior changes only by removing the deprecation warning, exactly as intended.

Explanation Nemotron 3 Super unverified

The original string literal `'\.'` contains an invalid backslash‑dot escape; Python emits a SyntaxWarning but still treats it as the two‑character string consisting of a backslash followed by a dot. Rewriting it as a raw string `r'\.'` removes the warning, yet the resulting value remains identical—a backslash and a dot—so the program’s observable behavior does not change; only the diagnostic message differs. Thus the fix is safe regarding semantics, though it alters the warning output.

Reproduce

file

prog.py (the before program)

minimized program

r1-p007.py

evidence sheet

r1-p007.md

regression fixture

r1-p007.py the minimized program with a header stating the observed change, for the tool's test suite

python3 prog.py
ruff check --preview --isolated --select W605 --fix prog.py
python3 prog.py

Upstream search GitHub issues Tavily Nemotron 3 Super + Ultra

W605 fix · W605 regex pattern backslash dot · invalid escape sequence warning fix · raw string literal prefix addition

• related astral-sh/ruff/issues/11491 Existing report is about W605 fix in f-strings, new finding is a plain string literal.
• duplicate astral-sh/ruff/issues/11461 Existing report 'W605 fix is unsafe' describes the same defect: autofix converts '\.' to raw string r'\.' which changes regex meaning.
• related astral-sh/ruff/issues/10434 W605 panic on f-string with backslashes, different construct (f-string vs plain string).
• related astral-sh/ruff/issues/6987 ANN401/W605 autofix error, but not the same raw-string behavior-change defect.
• related astral-sh/ruff/issues/5061 Feature request to use raw strings for W605 fix, which is what the new finding demonstrates going wrong.
• related astral-sh/ruff/pull/5352 PR implementing raw-string fix for W605, which introduced the defect in the new finding.

Details

program

Regex substitution backfire

hazard

When the string is used in a regex context via re.sub or similar, doubling backslashes changes the number of escape layers, potentially requiring compensatory changes elsewhere in the pattern.

oracle

stderr before: /w/prog.py:1: SyntaxWarning: invalid escape sequence '\.' stderr after: (none)

run

2026-10-17T20-50-05-853Z-2tuo-ruff-W605

--- orig.py	2026-10-17 16:50:21.723547676 -0400
+++ fixed.py	2026-10-17 16:50:21.933551754 -0400
@@ -1,5 +1,5 @@
 import re
-pattern = '\.'
+pattern = r'\.'
 text = 'a.b'
 result = re.sub(pattern, 'X', text)
 print(result)

# invalid-escape-sequence (W605)

Derived from the **pycodestyle** linter.

Fix is always available.

## What it does
Checks for invalid escape sequences.

## Why is this bad?
Invalid escape sequences are deprecated in Python 3.6.

## Example
```python
regex = "\.png$"
```

Use instead:
```python
regex = r"\.png$"
```

Or, if the string already contains a valid escape sequence:
```python
value = "new line\nand invalid escape \_ here"
```

Use instead:
```python
value = "new line\nand invalid escape \\_ here"
```

## References
- [Python documentation: String and Bytes literals](https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals)