JavaScript: Rephrase two predicates to help the optimiser.

javascript/ql/src/Security/CWE-116/IncompleteSanitization.ql

@@ -32,11 +32,19 @@ predicate isSimple(RegExpTerm t) {
   or
   isSimple(t.(RegExpGroup).getAChild())
   or
-  (
-    t instanceof RegExpAlt
-    or
-    t instanceof RegExpCharacterClass and not t.(RegExpCharacterClass).isInverted()
-  ) and
+  isSimpleCharacterClass(t)
+  or
+  isSimpleAlt(t)
+}
+
+/** Holds if `t` is a non-inverted character class that contains no ranges. */
+predicate isSimpleCharacterClass(RegExpCharacterClass t) {
+  not t.isInverted() and
+  forall(RegExpTerm ch | ch = t.getAChild() | isSimple(ch))
+}
+
+/** Holds if `t` is an alternation of simple terms. */
+predicate isSimpleAlt(RegExpAlt t) {
   forall(RegExpTerm ch | ch = t.getAChild() | isSimple(ch))
 }
 

javascript/ql/src/semmle/javascript/Regexp.qll

@@ -311,13 +311,19 @@ class RegExpSequence extends RegExpTerm, @regexp_seq {
     forall(RegExpTerm child | child = getAChild() | child.isNullable())
   }
 
-  language[monotonicAggregates]
   override string getConstantValue() {
-    // note: due to use of monotonic aggregates, this `strictconcat` will fail if
-    // `getConstantValue` is undefined for any child
-    result = strictconcat(RegExpTerm ch, int i | ch = getChild(i) |
-      ch.getConstantValue() order by i
-    )
+    result = getConstantValue(0)
+  }
+
+  /**
+   * Gets the single string matched by the `i`th child and all following children of
+   * this sequence, if any.
+   */
+  private string getConstantValue(int i) {
+    i = getNumChild() and
+    result = ""
+    or
+    result = getChild(i).getConstantValue() + getConstantValue(i+1)
   }
 }