diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
new file mode 100644
index 000000000..cd4d5feb0
--- /dev/null
+++ b/lib/shared/mention-utils.js
@@ -0,0 +1,29 @@
+// @flow
+
+import type { RelativeMemberInfo } from '../types/thread-types';
+import { oldValidUsernameRegexString } from './account-utils';
+import SearchIndex from './search-index';
+import { stringForUserExplicit } from './user-utils';
+
+function getTypeaheadUserSuggestions(
+  userSearchIndex: SearchIndex,
+  usersInThread: $ReadOnlyArray<RelativeMemberInfo>,
+  typedPrefix: string,
+): $ReadOnlyArray<RelativeMemberInfo> {
+  const userIDs = userSearchIndex.getSearchResults(typedPrefix);
+
+  return usersInThread
+    .filter(
+      (user: RelativeMemberInfo) =>
+        typedPrefix.length === 0 || userIDs.includes(user.id),
+    )
+    .sort((userA, userB) =>
+      stringForUserExplicit(userA).localeCompare(stringForUserExplicit(userB)),
+    );
+}
+
+const mentionRegex: RegExp = new RegExp(
+  `(^|.* )@(${oldValidUsernameRegexString})?$`,
+);
+
+export { getTypeaheadUserSuggestions, mentionRegex };
diff --git a/lib/shared/user-utils.js b/lib/shared/user-utils.js
index c97e9c404..3239f37da 100644
--- a/lib/shared/user-utils.js
+++ b/lib/shared/user-utils.js
@@ -1,52 +1,60 @@
 // @flow
 
 import bots from '../facts/bots';
 import staff from '../facts/staff';
 import type {
   ServerThreadInfo,
   RawThreadInfo,
   RelativeMemberInfo,
   ThreadInfo,
 } from '../types/thread-types';
 import type {
   RelativeUserInfo,
   UserInfo,
   UserInfos,
 } from '../types/user-types';
 import { memberHasAdminPowers } from './thread-utils';
 
 function stringForUser(user: RelativeUserInfo | RelativeMemberInfo): string {
   if (user.isViewer) {
     return 'you';
-  } else if (user.username) {
+  }
+
+  return stringForUserExplicit(user);
+}
+
+function stringForUserExplicit(
+  user: RelativeUserInfo | RelativeMemberInfo,
+): string {
+  if (user.username) {
     return user.username;
   } else {
     return 'anonymous';
   }
 }
 
 function isStaff(userID: string): boolean {
   if (staff.includes(userID)) {
     return true;
   }
   for (const key in bots) {
     const bot = bots[key];
     if (userID === bot.userID) {
       return true;
     }
   }
   return false;
 }
 
 function getKeyserverAdmin(
   community: ThreadInfo | RawThreadInfo | ServerThreadInfo,
   userInfos: UserInfos,
 ): ?UserInfo {
   // This hack only works as long as there is only one admin
   // Linear task to revert this:
   // https://linear.app/comm/issue/ENG-1707/revert-fix-getting-the-keyserver-admin-info
   const admin = community.members.find(memberHasAdminPowers);
   return admin ? userInfos[admin.id] : undefined;
 }
 
-export { stringForUser, isStaff, getKeyserverAdmin };
+export { stringForUser, stringForUserExplicit, isStaff, getKeyserverAdmin };