SmartChineseAnalyzer indexes whitespace characters as terms: VERTICAL TAB, FORM FEED, OGHAM SPACE MARK and the eleven spaces U+2000-U+200A (EN QUAD to HAIR SPACE, among them EN, EM and THIN SPACE) each become a one-character term, because Utility.getCharType classifies them OTHER instead of SPACE_LIKE

Finding LFV-728Area: analysis-smartcn / HHMM segmenter character classes (Utility.getCharType SPACE_LIKE test -> HHMMSegmenter OTHER branch -> one-character STRING term; SmartChineseAnalyzer and HMMChineseTokenizer)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-728

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 text with the stock SmartChineseAnalyzer, whose segmenter has a character class for 'Characters that act as a space' (CharType.SPACE_LIKE) and skips those characters, so you expect any space to separate words without a trace, as an ordinary space, a tab or the ideographic space U+3000 does. Instead Utility.getCharType knows only those five, and VERTICAL TAB, FORM FEED, OGHAM SPACE MARK and U+2000-U+200A fall through to OTHER and are each indexed as a one-character term: 'hello<THIN SPACE>world' gives [hello, U+2009, world], a TermQuery for U+2009 alone finds the document, and '中<U+2009>国' gives [中, U+2009, 国] where '中国' gives [中国]. So the exact phrase "hello world" matches only 3 of 7 documents that differ only in the separator (space, tab and U+3000 match; FORM FEED, U+1680, EM SPACE and THIN SPACE miss), and HTML indented with &emsp;&emsp; gets two whitespace terms, with no error.

Expected vs actual

Expected: hello<U+2009>world analyzes to [hello, world], like hello world.
Actual: It analyzes to [hello, U+2009, world]: term 1 expected:<[world]> but was:<[ ]>.

Files

src/org/apache/lucene/analysis/cn/smart/TestUnicodeSpacesAreSkipped.java (JUnit test)

Run it

Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
Tested on Lucene main a9779ef3c193 (2026-10-26): reproduced.
Both tests also treat U+00A0 NO-BREAK SPACE as a space: smartcn removes it like punctuation, with a position gap, which is tested design and not part of this finding (the finding page's TestSmartChineseThinSpace.java is the minimal test).
Class path: analysis-smartcn depends on the analysis-common module (SegmentingTokenizerBase), so the recipe adds lucene-analysis-common to the class path before compiling.
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:$LUCENE/lucene/analysis/smartcn/build/libs/lucene-analysis-smartcn-$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)

CP=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar   # analysis-smartcn needs analysis-common

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.cn.smart.TestUnicodeSpacesAreSkipped

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).

Files shown on the finding page

TestSmartChineseThinSpace.java

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.