diff --git a/src/items.js b/src/items.js
index 8d03e6c..a3ee9d7 100644
--- a/src/items.js
+++ b/src/items.js
@@ -33,13 +33,24 @@ export default {
     Zotero.debug(operation, tags);
     let updateCount = 0;
     for (let i = 0; i < tags.length; ++i) {
-      if (operation === "add" && !item.hasTag(tags[i])) {
+      const expTagRegex = /^('|"|`)(.+)\[(.+)\]\1$/;
+      let isCondition = true;
+      if (expTagRegex.test(tags[i])) {
+          let [tag, condition_tag] = tags[i].match(expTagRegex).slice(2);
+          tags[i] = tag;
+          if (condition_tag.startsWith("!")) {
+            isCondition = !item.hasTag(condition_tag.slice(1));
+          } else {
+            isCondition = item.hasTag(condition_tag);
+          }
+      }
+      if (operation === "add" && !item.hasTag(tags[i]) && isCondition) {
         item.addTag(tags[i], userTag ? 0 : 1);
         updateCount += 1;
-      } else if (operation === "remove" && item.hasTag(tags[i])) {
+      } else if (operation === "remove" && item.hasTag(tags[i]) && isCondition) {
         item.removeTag(tags[i]);
         updateCount += 1;
-      } else if (operation === "change") {
+      } else if (operation === "change" && isCondition) {
         if (item.hasTag(tags[i])) {
           item.removeTag(tags[i]);
         } else {