ICUFoldingFilter removes the hook, stroke or descender from older Cyrillic letters such as ҧ (U+04A7) but from none of the 24 letters of that kind added in Unicode 4.1-7.0, such as its modern Abkhaz counterpart ԥ (U+0525), so the two spellings of a word give different terms

Finding LFV-877Area: analysis / icu (ICUFoldingFilter UTR#30 folding data: the hand-written 'Rule: verbatim' Cyrillic block of DiacriticFolding.txt, compiled into utr30.nrm)Priority: lowImpact: minorNovelty: new

Finding page for LFV-877

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You index Abkhaz text in the older spelling 'аҧсуа' with ICUFoldingFilter, whose javadoc lists 'Diacritic removal (including stroke, hook, descender)' and which folds ҧ (U+04A7) to п, and you expect the modern spelling 'аԥсуа', written with ԥ (U+0525, the modern-orthography counterpart of U+04A7), to find it. Instead the hand-copied Cyrillic table in DiacriticFolding.txt, taken from the 2004 UTR#30 draft data for Unicode 4.0 and never extended, has no line for ԥ or for any of the other 23 hook, stroke or descender letters added in Unicode 4.1-7.0 (ӷ ӻ ӽ ӿ ԓ ԡ ԣ ꚋ ԧ ԩ ԯ and the capitals), so they are only case-folded and keep the mark, and the query finds 0 documents. All 48 older letters of the same kind lose the mark (46 fold to their base letter; ҷ/Ҷ fold to a wrong letter, the separate bug LFV-855).

The test shows that ICUFoldingFilter leaves the descender on Cyrillic ԥ (U+0525 PE WITH DESCENDER, Unicode 5.2, modern Abkhaz), while it folds the older form ҧ (U+04A7 PE WITH MIDDLE HOOK) to п.

Expected vs actual

Expected ԥ to fold to п (U+043F); actual: ԥ unchanged (term 0 expected:<[п]> but was:<[ԥ]>), with -ea and -da.

Files

• src/org/apache/lucene/analysis/icu/TestICUFoldingPeWithDescender.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/analysis/icu/build/libs/lucene-analysis-icu-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.analysis.icu.TestICUFoldingPeWithDescender

The module(s) analysis-icu also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.