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 { stringForUserExplicit } from './user-utils';
+
+function getTypeaheadUserSuggestions(
+  usersInThread: $ReadOnlyArray<RelativeMemberInfo>,
+  typedPrefix: string,
+): $ReadOnlyArray<RelativeMemberInfo> {
+  return usersInThread
+    .filter(user => {
+      if (stringForUserExplicit(user) === 'anonymous') {
+        return false;
+      }
+      return stringForUserExplicit(user).startsWith(typedPrefix);
+    })
+    .sort((userA, userB) => {
+      return stringForUserExplicit(userA).localeCompare(
+        stringForUserExplicit(userB),
+      );
+    });
+}
+
+const mentionRegex: RegExp = new RegExp(
+  `(^|.* )@(${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
@@ -25,6 +25,16 @@
   }
 }
 
+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;
@@ -49,4 +59,4 @@
   return admin ? userInfos[admin.id] : undefined;
 }
 
-export { stringForUser, isStaff, getKeyserverAdmin };
+export { stringForUser, stringForUserExplicit, isStaff, getKeyserverAdmin };