Java: add RegexpCheckBarrier to trust-boundary-violation sanitizers

The trust-boundary-violation query only recognized OWASP ESAPI validators
as sanitizers. ESAPI is rarely used in modern Java projects, while regex
validation via String.matches() and @javax.validation.constraints.Pattern
is the standard approach in Spring/Jakarta applications.

RegexpCheckBarrier already exists in Sanitizers.qll and is used by other
queries (e.g., RequestForgery). This wires it into TrustBoundaryConfig,
so patterns like input.matches("[a-zA-Z0-9]+") and @Pattern annotations
are recognized as sanitizers, consistent with the existing ESAPI treatment.

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.

java/ql/lib/semmle/code/java/security/TrustBoundaryViolationQuery.qll

@@ -40,7 +40,8 @@ module TrustBoundaryConfig implements DataFlow::ConfigSig {
   predicate isBarrier(DataFlow::Node node) {
     node instanceof TrustBoundaryValidationSanitizer or
     node.getType() instanceof HttpServletSession or
-    node instanceof SimpleTypeSanitizer
+    node instanceof SimpleTypeSanitizer or
+    node instanceof RegexpCheckBarrier
   }
 
   predicate isSink(DataFlow::Node sink) { sink instanceof TrustBoundaryViolationSink }

java/ql/test/query-tests/security/CWE-501/TrustBoundaryViolations.java

@@ -31,5 +31,11 @@ public class TrustBoundaryViolations extends HttpServlet {
             }
         } catch (Exception e) {
         }
+
+        // GOOD: Bean Validation @Pattern annotation constrains the input via regex.
+        String input4 = request.getParameter("input4");
+        if (input4.matches("[a-zA-Z0-9]+")) {
+            request.getSession().setAttribute("input4", input4);
+        }
     }
 }