ESQL: Workaround for RLike handling of empty lang pattern

docs/changelog/128895.yaml

@@ -0,0 +1,6 @@
+pr: 128895
+summary: Workaround for RLike handling of empty lang pattern
+area: ES|QL
+type: bug
+issues:
+ - 128813

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/predicate/regex/AbstractStringPattern.java

@@ -32,7 +32,11 @@ public boolean matchesAll() {
 
  @Override
  public String exactMatch() {
- IntsRef singleton = Operations.getSingleton(automaton());
+ Automaton a = automaton();
+ if (a.getNumStates() == 0) { // workaround for https://github.com/elastic/elasticsearch/pull/128887
+ return null; // Empty automaton has no matches
+ }
+ IntsRef singleton = Operations.getSingleton(a);
  return singleton != null ? UnicodeUtil.newString(singleton.ints, singleton.offset, singleton.length) : null;
  }
 }

x-pack/plugin/esql-core/src/test/java/org/elasticsearch/xpack/esql/core/expression/predicate/regex/StringPatternTests.java

@@ -33,8 +33,12 @@ private boolean rlikeMatchesAll(String pattern) {
  return rlike(pattern).matchesAll();
  }
 
+ private String exactMatchRLike(String pattern) {
+ return rlike(pattern).exactMatch();
+ }
+
  private boolean rlikeExactMatch(String pattern) {
- return pattern.equals(rlike(pattern).exactMatch());
+ return pattern.equals(exactMatchRLike(pattern));
  }
 
  public void testWildcardMatchAll() {
@@ -86,4 +90,20 @@ public void testRegexExactMatch() {
  assertTrue(rlikeExactMatch("abc"));
  assertTrue(rlikeExactMatch("12345"));
  }
+
+ public void testRegexExactMatchWithEmptyMatch() {
+ // As soon as there's one no conditional `#` in the pattern, it'll match nothing
+ assertNull(exactMatchRLike("#"));
+ assertNull(exactMatchRLike("##"));
+ assertNull(exactMatchRLike("#foo"));
+ assertNull(exactMatchRLike("#foo#"));
+ assertNull(exactMatchRLike("f#oo"));
+ assertNull(exactMatchRLike("foo#"));
+ assertNull(exactMatchRLike("#[A-Z]*"));
+ assertNull(exactMatchRLike("foo(#)"));
+
+ assertNotNull(exactMatchRLike("foo#?"));
+ assertNotNull(exactMatchRLike("#|foo"));
+ assertNotNull(exactMatchRLike("foo|#"));
+ }
 }

x-pack/plugin/esql-core/src/test/java/org/elasticsearch/xpack/esql/core/util/TestUtils.java

@@ -66,8 +66,8 @@ public static String stripThrough(String input) {
  /** Returns the input string, but with parts of it having the letter casing changed. */
  public static String randomCasing(String input) {
  StringBuilder sb = new StringBuilder(input.length());
- for (int i = 0, inputLen = input.length(), step = (int) Math.sqrt(inputLen), chunkEnd; i < inputLen; i += step) {
- chunkEnd = Math.min(i + step, inputLen);
+ for (int i = 0, inputLen = input.length(), step = (int) Math.sqrt(inputLen); i < inputLen; i += step) {
+ var chunkEnd = Math.min(i + step, inputLen);
  var chunk = input.substring(i, chunkEnd);
  sb.append(randomBoolean() ? chunk.toLowerCase(Locale.ROOT) : chunk.toUpperCase(Locale.ROOT));
  }

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/RestEsqlTestCase.java

@@ -1440,6 +1440,46 @@ public void testReplaceStringCasingWithInsensitiveWildcardMatch() throws IOExcep
  assertThat(answer.get("values"), equalTo(List.of(List.of("_\"_$_(_)_+_._[_]_^_{_|_}___", "_#_&_<_>___"))));
  }
 
+ public void testRLikeHandlingOfEmptyLanguagePattern() throws IOException {
+ createIndex(testIndexName(), Settings.EMPTY, """
+ {
+ "properties": {
+ "field": {
+ "type": "keyword"
+ }
+ }
+ }
+ """);
+ for (var val : List.of("#", "foo#bar")) {
+ Request doc = new Request("POST", testIndexName() + "/_doc?refresh=true");
+ doc.setJsonEntity("""
+ {
+ "field": "%s"
+ }
+ """.formatted(val));
+ client().performRequest(doc);
+ }
+ // pushed down, matches nothing
+ var query = "FROM " + testIndexName() + " | WHERE TO_LOWER(field) RLIKE \"#\"";
+ var answer = runEsql(requestObjectBuilder().query(query));
+ assertThat(answer.get("values"), equalTo(List.of()));
+
+ // matches nothing
+ query = "FROM " + testIndexName() + " | WHERE field RLIKE \"#\"";
+ answer = runEsql(requestObjectBuilder().query(query));
+ assertThat(answer.get("values"), equalTo(List.of()));
+
+ // matches one doc
+ query = "FROM " + testIndexName() + " | WHERE field RLIKE \"\\\\#\"";
+ answer = runEsql(requestObjectBuilder().query(query));
+ assertThat(answer.get("values"), equalTo(List.of(List.of("#"))));
+
+ // matches both docs
+ query = "FROM " + testIndexName() + " | WHERE field RLIKE \".*\\\\#.*\" | SORT field";
+ answer = runEsql(requestObjectBuilder().query(query));
+ assertThat(answer.get("values"), equalTo(List.of(List.of("#"), List.of("foo#bar"))));
+ }
+
  protected static Request prepareRequestWithOptions(RequestObjectBuilder requestObject, Mode mode) throws IOException {
  requestObject.build();
  Request request = prepareRequest(mode);

x-pack/plugin/esql/qa/testFixtures/src/main/resources/where-like.csv-spec

@@ -423,3 +423,13 @@ emp_no:integer |first_name:keyword
 10001 |Georgi
 10055 |Georgy
 ;
+
+# test for https://github.com/elastic/elasticsearch/issues/128813
+rlikeWithEmptyLanguagePattern
+required_capability: rlike_with_empty_language_pattern
+ROW x = "abc" | EVAL bool = x RLIKE "#"
+;
+
+x:keyword | bool:boolean
+abc | false
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -1177,6 +1177,11 @@ public enum Cap {
  */
  ENABLE_LOOKUP_JOIN_ON_ALIASES,
 
+ /**
+ * Allows RLIKE to correctly handle the "empty language" flag, `#`.
+ */
+ RLIKE_WITH_EMPTY_LANGUAGE_PATTERN,
+
  /**
  * MATCH PHRASE function
  */