diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
new file mode 100644
--- /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
--- a/lib/shared/user-utils.js
+++ b/lib/shared/user-utils.js
@@ -18,7 +18,15 @@
 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';
@@ -49,4 +57,4 @@
   return admin ? userInfos[admin.id] : undefined;
 }
 
-export { stringForUser, isStaff, getKeyserverAdmin };
+export { stringForUser, stringForUserExplicit, isStaff, getKeyserverAdmin };