diff --git a/lib/shared/markdown.js b/lib/shared/markdown.js
--- a/lib/shared/markdown.js
+++ b/lib/shared/markdown.js
@@ -2,7 +2,7 @@
 
 import invariant from 'invariant';
 
-import { oldValidUsernameRegexString } from './account-utils.js';
+import { markdownMentionRegex } from './mention-utils.js';
 import type { RelativeMemberInfo } from '../types/thread-types.js';
 
 // simple-markdown types
@@ -90,8 +90,6 @@
 
 const urlRegex: RegExp = /^(https?:\/\/[^\s<]+[^<.,:;"')\]\s])/i;
 
-const mentionRegex = new RegExp(`^(@(${oldValidUsernameRegexString}))\\b`);
-
 export type JSONCapture = Array<string> & {
   +json: Object,
   +index?: void,
@@ -205,18 +203,18 @@
     if (!state.inline) {
       return null;
     }
-    const result = mentionRegex.exec(source);
+    const result = markdownMentionRegex.exec(source);
     if (!result) {
       return null;
     }
     const username = result[2];
-    invariant(username, 'mentionRegex should match two capture groups');
+    invariant(username, 'markdownMentionRegex should match two capture groups');
     if (!memberSet.has(username.toLowerCase())) {
       return null;
     }
     return result;
   };
-  match.regex = mentionRegex;
+  match.regex = markdownMentionRegex;
   return match;
 }
 
diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
--- a/lib/shared/mention-utils.js
+++ b/lib/shared/mention-utils.js
@@ -1,5 +1,6 @@
 // @flow
 
+import { oldValidUsernameRegexString } from './account-utils.js';
 import SearchIndex from './search-index.js';
 import { threadOtherMembers } from './thread-utils.js';
 import { stringForUserExplicit } from './user-utils.js';
@@ -15,6 +16,12 @@
   +end: number,
 };
 
+// The simple-markdown package already breaks words out for us, and we are
+// supposed to only match when the first word of the input matches
+const markdownMentionRegex: RegExp = new RegExp(
+  `^(@(${oldValidUsernameRegexString}))\\b`,
+);
+
 function isMentioned(username: string, text: string): boolean {
   return new RegExp(`\\B@${username}\\b`, 'i').test(text);
 }
@@ -73,6 +80,7 @@
 }
 
 export {
+  markdownMentionRegex,
   isMentioned,
   getTypeaheadUserSuggestions,
   getNewTextAndSelection,