diff --git a/org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/parser/ComplianceDiagnoseTest.java b/org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/parser/ComplianceDiagnoseTest.java
index 181e73a3dab..14130911bd6 100644
--- a/org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/parser/ComplianceDiagnoseTest.java
+++ b/org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/parser/ComplianceDiagnoseTest.java
@@ -3234,6 +3234,9 @@ public void testBug399781() {
 	};
 	String usLevel = this.complianceLevel < ClassFileConstants.JDK9 ? "WARNING" : "ERROR";
 	String errorMessage = this.complianceLevel < ClassFileConstants.JDK9 ? "\'_\' should not be used as an identifier, since it is a reserved keyword from source level 1.8 on\n" : "\'_\' is a keyword from source level 9 onwards, cannot be used as identifier\n";
+	if (this.complianceLevel >= ClassFileConstants.JDK21) {
+		errorMessage = "Unnamed Patterns and Variables is a preview feature and disabled by default. Use --enable-preview to enable\n";
+	}
 	String expectedProblemLog =
 			"----------\n" +
 			"1. " + usLevel +" in X.java (at line 2)\n" +
diff --git a/org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/complete/CompletionParser.java b/org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/complete/CompletionParser.java
index 16ccdcb9b8d..4c200b27c80 100644
--- a/org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/complete/CompletionParser.java
+++ b/org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/complete/CompletionParser.java
@@ -4232,6 +4232,7 @@ && isIndirectlyInsideFieldInitialization()) { // enum initializers indeed need m
 					case TokenNamesuper: // e.g. super[.]fred()
 						this.invocationType = SUPER_RECEIVER;
 						break;
+					case TokenNameUNDERSCORE:
 					case TokenNameIdentifier: // e.g. bar[.]fred()
 						if (topKnownElementKind(COMPLETION_OR_ASSIST_PARSER) != K_BETWEEN_NEW_AND_LEFT_BRACKET) {
 							if (this.identifierPtr != prevIdentifierPtr) { // if identifier has been consumed, e.g. this.x[.]fred()
@@ -4246,6 +4247,7 @@ && isIndirectlyInsideFieldInitialization()) { // enum initializers indeed need m
 			case TokenNameCOLON_COLON:
 				this.inReferenceExpression = true;
 				break;
+			case TokenNameUNDERSCORE:
 			case TokenNameIdentifier:
 				if (this.inReferenceExpression)
 					break;
@@ -4317,6 +4319,7 @@ && isInsideSwitch() && checkYieldKeyword()) {
 					this.qualifier = this.expressionPtr; // remenber the last expression so that arguments are correctly computed
 				}
 				switch (previous) {
+					case TokenNameUNDERSCORE:
 					case TokenNameIdentifier: // e.g. fred[(]) or foo.fred[(])
 						if (topKnownElementKind(COMPLETION_OR_ASSIST_PARSER) == K_SELECTOR) {
 							int info = 0;
@@ -4436,6 +4439,7 @@ && isInsideSwitch() && checkYieldKeyword()) {
 					pushOnElementStack(K_BETWEEN_LEFT_AND_RIGHT_BRACKET);
 				} else {
 					switch (previous) {
+						case TokenNameUNDERSCORE:
 						case TokenNameIdentifier:
 						case TokenNameboolean:
 						case TokenNamebyte: