HTMLStripCharFilter leaves hexadecimal character references with six hex digits (􏿽 􀀀 the zero-padded &#x0000E9) undecoded, because its length check counts the leading 'x' and so lets only five hex digits through

Finding LFV-289Area: analysis / HTMLStripCharFilter (JFlex grammar, numeric character references; also the 'htmlStrip' char filter SPI)Priority: lowImpact: minorNovelty: new

Finding page for LFV-289

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

What it shows

You run HTML through HTMLStripCharFilter and expect every numeric character reference up to U+10FFFF to be decoded, as the scanner's own comment says ('10FFFF: max 6 hex chars') and as its decimal branch does for 􏿿 and the zero-padded é. Instead a hex reference with six hex digits, such as 􏿽 􀀀 or é stays in the output as literal text: the length that is checked includes the leading 'x', so only five hex digits pass. Through StandardTokenizer, 'bad' is indexed as the tokens b, x000061 and d, and a TermQuery for 'bad' does not find the document.

The test shows that HTMLStripCharFilter leaves a hexadecimal character reference with six hex digits undecoded (its length check counts the leading x).

Expected vs actual

Expected é to strip to é; actual: the literal é (expected:<[é]> but was:<[é]>), with -ea and -da.

Files

src/org/apache/lucene/analysis/charfilter/TestHTMLStripHexCharRefSixDigits.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.charfilter.TestHTMLStripHexCharRefSixDigits

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.