Refactor regex sanitizer

Move it to Sanitizers.qll and rename it to be more general (mostly
following Go).

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

@@ -166,22 +166,7 @@ private class HostComparisonSanitizer extends RequestForgerySanitizer {
 }
 
 /**
- * A qualifier in a call to a `.matches()` method that is a sanitizer for URL redirects.
+ * A comparison with a regular expression that is a sanitizer for URL redirects.
- *
- * Matches any method call where the method is named `matches`.
  */
-private predicate isMatchesSanitizer(Guard guard, Expr e, boolean branch) {
+private class RegexpCheckRequestForgerySanitizer extends RequestForgerySanitizer instanceof RegexpCheckBarrier
-  guard =
+{ }
-    any(MethodCall method |
-      method.getMethod().getName() = "matches" and
-      e = method.getQualifier() and
-      branch = true
-    )
-}
-
-/**
- * A qualifier in a call to `.matches()` that is a sanitizer for URL redirects.
- */
-private class MatchesSanitizer extends RequestForgerySanitizer {
-  MatchesSanitizer() { this = DataFlow::BarrierGuard<isMatchesSanitizer/3>::getABarrierNode() }
-}

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

@@ -3,6 +3,7 @@ overlay[local?]
 module;
 
 import java
+private import semmle.code.java.controlflow.Guards
 private import semmle.code.java.dataflow.DataFlow
 
 /**
@@ -29,3 +30,31 @@ class SimpleTypeSanitizer extends DataFlow::Node {
     this.getType() instanceof EnumType
   }
 }
+
+/**
+ * Holds if `guard` holds with branch `branch` if `e` matches a regular expression.
+ *
+ * This is overapproximate: we do not attempt to reason about the correctness of the regexp.
+ *
+ * Use this if you want to define a derived `DataFlow::BarrierGuard` without
+ * make the type recursive. Otherwise use `RegexpCheckBarrier`.
+ */
+predicate regexpMatchGuardChecks(Guard guard, Expr e, boolean branch) {
+  guard =
+    any(MethodCall method |
+      method.getMethod().getName() = "matches" and
+      e = method.getQualifier() and
+      branch = true
+    )
+}
+
+/**
+ * A check against a regular expression, considered as a barrier guard.
+ *
+ * This is overapproximate: we do not attempt to reason about the correctness of the regexp.
+ */
+class RegexpCheckBarrier extends DataFlow::Node {
+  RegexpCheckBarrier() {
+    this = DataFlow::BarrierGuard<regexpMatchGuardChecks/3>::getABarrierNode()
+  }
+}