diff --git a/lib/shared/account-utils.js b/lib/shared/account-utils.js
--- a/lib/shared/account-utils.js
+++ b/lib/shared/account-utils.js
@@ -53,7 +53,6 @@
 
 export {
   usernameMaxLength,
-  oldValidUsernameRegexString,
   validUsernameRegex,
   oldValidUsernameRegex,
   validEmailRegex,
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
@@ -2,7 +2,6 @@
 
 import * as React from 'react';
 
-import { oldValidUsernameRegexString } from './account-utils.js';
 import SentencePrefixSearchIndex from './sentence-prefix-search-index.js';
 import { threadOtherMembers } from './thread-utils.js';
 import { stringForUserExplicit } from './user-utils.js';
@@ -49,8 +48,9 @@
 
 // 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 markdownUserMentionRegexString = '[a-zA-Z0-9-_.]+';
 const markdownUserMentionRegex: RegExp = new RegExp(
-  `^(@(${oldValidUsernameRegexString}))\\b`,
+  `^(@(${markdownUserMentionRegexString}))\\b`,
 );
 
 function isUserMentioned(username: string, text: string): boolean {
@@ -58,7 +58,7 @@
 }
 
 const userMentionsExtractionRegex = new RegExp(
-  `\\B(@(${oldValidUsernameRegexString}))\\b`,
+  `\\B(@(${markdownUserMentionRegexString}))\\b`,
   'g',
 );