Monitor with RegexpQueryHandler silently misses regexp matches on any short token that contains a supplementary character (an emoji, a CJK Extension B ideograph, a Gothic or Adlam letter), because SuffixingNGramTokenFilter emits no ngrams for that token

Finding LFV-347Area: monitor / presearcher (RegexpQueryHandler document-side ngram filter SuffixingNGramTokenFilter)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-347

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

What it shows

You build a Monitor whose presearcher uses RegexpQueryHandler, register the query /hel.*/ and match a document whose text is 'hello😀' (WhitespaceAnalyzer, one token). You expect a match, because the regexp matches that token and the presearcher may only skip queries that cannot match. Instead SuffixingNGramTokenFilter, which should emit every substring of the token with the suffix 'XX' so that the query's index term 'helXX' is found, starts its gram size at the token's UTF-16 length (7) but checks it against the token's code-point count (6), gives up on the token without emitting a single ngram, and the query is silently never run.

The test shows that a Monitor with RegexpQueryHandler never runs a regexp query against a token that contains a supplementary character, because SuffixingNGramTokenFilter emits no ngrams for that token.

Expected vs actual

Expected: /hel.*/ matches the document whose only token is "hello" + U+1F600 (1 match). Actual (-ea and -da): 0 matches, and 0 queries are run by the presearcher.

Files

• src/org/apache/lucene/monitor/TestRegexpQueryHandlerSupplementaryChar.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:$LUCENE/lucene/memory/build/libs/lucene-memory-$V.jar
CP=$CP:$LUCENE/lucene/monitor/build/libs/lucene-monitor-$V.jar
CP=$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.monitor.TestRegexpQueryHandlerSupplementaryChar

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.