GeezerVison

csv.Sniffer detects a doubled-quote dialect with a pattern built from two
greedy character classes, both of which admit the quote character:

%(quote)s[^%(delim)s\n]*%(quote)s[^%(delim)s\n]*%(quote)s

Because either class can absorb a quote, the engine has to try every way of
splitting a run of quotes between them. On quote-heavy input that blows up:
sniffing the sample from the issue with 60 iterations takes about 9.7 s on 3.15.

Excluding the quote from the first class forces the quote that follows it to
match at the first one available, which removes the ambiguity:

%(quote)s[^%(delim)s%(quote)s\n]*%(quote)s[^%(delim)s\n]*%(quote)s

The second class has to keep admitting the quote — that is what lets "a""b"
match, where it consumes "b so the third quote can land on the closing one.
Narrowing both classes, or making the first one atomic while it still admits the
quote (as GH-109639 did), makes the pattern unmatchable and silently disables
detection; test_doublequote catches both.

This is the change @serhiy-storchaka described in #109638. It is not needed on
main, where these regexes no longer exist after the Sniffer rewrite in
gh-83273, so this PR is based directly on 3.15. 3.14 and 3.13 are affected
too and will get the same fix once this is reviewed.

Measurements

csv.Sniffer().sniff() on the sample from the issue body, run on 3.15.0b1:

Growth is still superlinear, as noted in the issue — this narrows the search
rather than replacing the approach — but it is far slower.

Test

test_sniff_regex_backtracking mirrors the test added on main, with one
difference: it uses 60 iterations rather than 100. main can afford 100 because
the rewrite made the cost negligible, whereas with this targeted fix 100
iterations still takes about 1.7 s, which is too slow to put in the suite. 60 is
the size quoted in the issue and costs 0.23 s.

Whole-file test_csv.py on 3.15.0b1: 0.45 s with the fix, 11.6 s
without it, 135 passing either way — so the new test acts as the same kind of
canary it does on main rather than asserting on a timer.

Checking the change is behaviour-preserving

Beyond the suite, I compared sniff() results between patched and unpatched
csv.py across 8726 cases — the delimiter/quote/doublequote/skipinitialspace
combinations, TestSniffer's own corpus, and 4000 randomized quote-dense
strings — with no divergence in the returned dialect or the exceptions raised.

Disclosure: prepared with AI assistance (Claude Code) — used to analyse the
backtracking behaviour, produce the measurements and differential check above,
and draft the patch and this description.

• Issue: csv.Sniffer regular expression has significant backtracing #109638