Fix regex per suggestion

Author: JarLob

javascript/ql/src/experimental/semmle/javascript/Actions.qll

@@ -307,7 +307,10 @@ module Actions {
       // not just the last (greedy match) or first (reluctant match).
       // TODO: This only handles expression strings that refer to contexts.
       // It does not handle operators within the expression.
-      result = this.getValue().regexpFind("[A-Za-z0-9_\\.\\-]+(?=\\s*\\}\\})", _, _)
+      result =
+        this.getValue()
+            .regexpFind("\\$\\{\\{\\s*[A-Za-z0-9_\\.\\-]+\\s*\\}\\}", _, _)
+            .regexpCapture("\\$\\{\\{\\s*([A-Za-z0-9_\\.\\-]+)\\s*\\}\\}", 1)
     }
   }
 }