ArabicStemmer removes at most one prefix, so after it strips the conjunction waw it never strips the article that follows (wa+bi-al-, wa+li-l-, wa+ka-al-, wa+fa-al-), unlike the light10 algorithm ArabicAnalyzer documents

Finding LFV-665Area: analysis-common / Arabic light stemmer (ArabicStemmer.stemPrefix; ArabicStemFilter, ArabicAnalyzer and the 'arabicStem' factory)Priority: lowImpact: silent wrong resultsNovelty: new

Finding page for LFV-665

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 Arabic text with the stock ArabicAnalyzer, whose javadoc says it implements light stemming 'as specified by' the light10 paper; that paper removes a leading waw ('and') first and then a definite article, so 'وبالكتاب' ('and with the book') should stem to 'كتاب' ('book'), like 'الكتاب' and 'بالكتاب'. Instead ArabicStemmer.stemPrefix scans one list (the six articles, then waw) and returns after the first match, so once it has removed the waw it never tries the articles: 'وبالكتاب' becomes 'بالكتاب' and 'وللطلاب' becomes 'للطلاب'. A TermQuery for the analyzed form of 'الكتاب' then finds 0 of 1 documents that contain only 'وبالكتاب', with no error.

The test shows that ArabicStemFilter removes the conjunction waw from وبالكتاب ("and with the book") but then not the article بال, unlike light10 (waw first, then the article).

Expected vs actual

Expected كتاب; actual بالكتاب (term 0 expected:<[]كتاب> but was:<[بال]كتاب>), with -ea and -da.

Files

• src/org/apache/lucene/analysis/ar/TestArabicWawThenArticle.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.ar.TestArabicWawThenArticle

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.