CapitalizationFilter changes case one UTF-16 char at a time, so cased letters outside the BMP (Adlam, Osage, Deseret and five other scripts) are neither capitalized nor lower-cased, while BMP letters in the same token are

Finding LFV-533Area: analysis-common / miscellaneous (CapitalizationFilter case mapping)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-533

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

What it shows

You add CapitalizationFilter, documented to 'make the first letter capital and the rest lower case', and give it a word in a cased script outside the BMP, for example the Adlam word U+1E922 U+1E923 (two small letters), and expect U+1E900 U+1E923 (capital first letter). Instead every supplementary letter keeps its case: U+1E922 U+1E923 comes out unchanged, U+1E900 U+1E901 is not lower-cased to U+1E900 U+1E923, and 'aB' followed by the Osage capital U+104B0 gives 'Ab' + U+104B0, because processWord applies Character.toUpperCase(char) and toLowerCase(char) to single UTF-16 units and a surrogate unit maps to itself. BMP letters in the same token are mapped, nothing is thrown, and the onlyFirstWord and keep-word forceFirstLetter paths behave the same way.

The test shows that CapitalizationFilter does not capitalize a first letter outside the BMP: the Adlam word U+1E922 U+1E923 goes through KeywordTokenizer + CapitalizationFilter unchanged.

Expected vs actual

Expected U+1E900 U+1E923 (capital first letter); actual U+1E922 U+1E923 (expected:<[1e900, 1e923]> but was:<[1e922, 1e923]>), with -ea and -da.

Files

• src/org/apache/lucene/analysis/miscellaneous/TestCapitalizationFilterSupplementary.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: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.miscellaneous.TestCapitalizationFilterSupplementary

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.