diff --git a/java/ql/lib/change-notes/2026-04-04-trust-boundary-regexp-barrier.md b/java/ql/lib/change-notes/2026-04-04-trust-boundary-regexp-barrier.md
new file mode 100644
index 000000000000..b80c0611b6de
--- /dev/null
+++ b/java/ql/lib/change-notes/2026-04-04-trust-boundary-regexp-barrier.md
@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `java/trust-boundary-violation` query now recognizes regular expression checks (including `String.matches()` guards and `@javax.validation.constraints.Pattern` annotations) as sanitizers, consistent with the existing treatment of ESAPI validators. This reduces false positives when input is validated against a pattern before being stored in a session.
diff --git a/java/ql/lib/semmle/code/java/security/TrustBoundaryViolationQuery.qll b/java/ql/lib/semmle/code/java/security/TrustBoundaryViolationQuery.qll
index d234f3df20ce..78589add96dc 100644
--- a/java/ql/lib/semmle/code/java/security/TrustBoundaryViolationQuery.qll
+++ b/java/ql/lib/semmle/code/java/security/TrustBoundaryViolationQuery.qll
@@ -31,17 +31,26 @@ private class ExternalTrustBoundaryValidationSanitizer extends TrustBoundaryVali
   ExternalTrustBoundaryValidationSanitizer() { barrierNode(this, "trust-boundary-violation") }
 }
 
+private class SimpleTypeTrustBoundaryValidationSanitizer extends TrustBoundaryValidationSanitizer instanceof SimpleTypeSanitizer
+{ }
+
+private class RegexpCheckTrustBoundaryValidationSanitizer extends TrustBoundaryValidationSanitizer instanceof RegexpCheckBarrier
+{ }
+
+private class HttpServletSessionTypeTrustBoundaryValidationSanitizer extends TrustBoundaryValidationSanitizer
+{
+  HttpServletSessionTypeTrustBoundaryValidationSanitizer() {
+    this.getType() instanceof HttpServletSession
+  }
+}
+
 /**
  * Taint tracking for data that crosses a trust boundary.
  */
 module TrustBoundaryConfig implements DataFlow::ConfigSig {
   predicate isSource(DataFlow::Node source) { source instanceof TrustBoundaryViolationSource }
 
-  predicate isBarrier(DataFlow::Node node) {
-    node instanceof TrustBoundaryValidationSanitizer or
-    node.getType() instanceof HttpServletSession or
-    node instanceof SimpleTypeSanitizer
-  }
+  predicate isBarrier(DataFlow::Node node) { node instanceof TrustBoundaryValidationSanitizer }
 
   predicate isSink(DataFlow::Node sink) { sink instanceof TrustBoundaryViolationSink }
 
diff --git a/java/ql/test/query-tests/security/CWE-501/TrustBoundaryViolations.java b/java/ql/test/query-tests/security/CWE-501/TrustBoundaryViolations.java
index d676e3e96783..06e9c6cc929f 100644
--- a/java/ql/test/query-tests/security/CWE-501/TrustBoundaryViolations.java
+++ b/java/ql/test/query-tests/security/CWE-501/TrustBoundaryViolations.java
@@ -31,5 +31,19 @@ public void doGet(HttpServletRequest request, HttpServletResponse response) {
             }
         } catch (Exception e) {
         }
+
+        // GOOD: A direct String.matches(...) regex check constrains the input before it is written to the session.
+        String input4 = request.getParameter("input4");
+        if (input4.matches("[a-zA-Z0-9]+")) {
+            request.getSession().setAttribute("input4", input4);
+        }
+    }
+
+    @javax.validation.constraints.Pattern(regexp = "^[a-zA-Z0-9]+$")
+    String validatedField;
+
+    public void doPost(HttpServletRequest request, HttpServletResponse response) {
+        // GOOD: The field is constrained by a @Pattern annotation.
+        request.getSession().setAttribute("validated", validatedField);
     }
 }
diff --git a/java/ql/test/query-tests/security/CWE-501/options b/java/ql/test/query-tests/security/CWE-501/options
index 37d627da7e82..15ba67d18321 100644
--- a/java/ql/test/query-tests/security/CWE-501/options
+++ b/java/ql/test/query-tests/security/CWE-501/options
@@ -1 +1 @@
-//semmle-extractor-options: --javac-args -cp ${testdir}/../../../stubs/esapi-2.0.1:${testdir}/../../../stubs/javax-servlet-2.5
+//semmle-extractor-options: --javac-args -cp ${testdir}/../../../stubs/esapi-2.0.1:${testdir}/../../../stubs/javax-servlet-2.5:${testdir}/../../../stubs/javax-validation-constraints