Flexible query parser: EscapeQuerySyntaxImpl does not escape '@' after the first character of a term or a term that is exactly && or ||, so QueryNode#toQueryString output for such terms does not parse

Finding LFV-398Area: queryparser / flexible parser term escaping (EscapeQuerySyntaxImpl, as used by QueryNode#toQueryString)Priority: lowImpact: minorNovelty: new

Finding page for LFV-398

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

What it shows

Since 9.1 the standard flexible parser reads an '@' anywhere in a term as syntax (it must be escaped, as in user\@example.com), and && and || are its AND and OR operators, so QueryNode#toQueryString, documented to return 'a query string understood by the query parser', has to escape them. Its escaper EscapeQuerySyntaxImpl escapes '@' only when it is the first character of a term and lists AND, OR and NOT but not && or || as reserved words, so the node for 'user\@example.com' prints 'df:user@example.com' and the nodes for '\&&' and '\||' print 'df:&&' and 'df:||'. Parsing any of these strings again throws INVALID_SYNTAX_CANNOT_PARSE; with a small fix to the escaper's tables every printable-ASCII term of length 1-3 round-trips.

The test shows that flexible QueryNode#toQueryString(new EscapeQuerySyntaxImpl()) does not escape an inner @ (user\@example.com) or the term && (\&&), so the printed query string does not parse.

Expected vs actual

Expected: df:user\@example.com and df:\&& (both parse back). Actual: df:user@example.com and df:&& (INVALID_SYNTAX_CANNOT_PARSE when re-parsed).

Files

Run it

# 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/facet/build/libs/lucene-facet-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:$LUCENE/lucene/queryparser/build/libs/lucene-queryparser-$V.jar
CP=$CP:$LUCENE/lucene/sandbox/build/libs/lucene-sandbox-$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.queryparser.flexible.standard.parser.TestEscapeQuerySyntaxRoundTrip

The module(s) facet also need their third-party runtime jars; ./gradlew -p lucene/<module> dependencies lists them.

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).

Formal models and scripts this finding's part of the hunt used: queryparser-flexible-simple-xml

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.