diff --git a/web/utils/typeahead-utils.test.js b/web/utils/typeahead-utils.test.js
--- a/web/utils/typeahead-utils.test.js
+++ b/web/utils/typeahead-utils.test.js
@@ -1,6 +1,9 @@
 // @flow
 
-import { getTypeaheadOverlayScroll } from './typeahead-utils.js';
+import {
+  getTypeaheadOverlayScroll,
+  webMentionTypeaheadRegex,
+} from './typeahead-utils.js';
 import { typeaheadStyle } from '../chat/chat-constants.js';
 
 describe('getTypeaheadOverlayScroll', () => {
@@ -30,3 +33,75 @@
       ),
   );
 });
+
+describe('webMentionTypeaheadRegex', () => {
+  const validMatches = [
+    {
+      text: '@mention123',
+      textPrefix: '',
+      mentionText: 'mention123',
+    },
+    {
+      text: '@mention with space',
+      textPrefix: '',
+      mentionText: 'mention with space',
+    },
+    {
+      text: 'text prefix @mention with space',
+      textPrefix: 'text prefix ',
+      mentionText: 'mention with space',
+    },
+    {
+      text: 'This is a test @mention',
+      textPrefix: 'This is a test ',
+      mentionText: 'mention',
+    },
+    {
+      text: '@',
+      textPrefix: '',
+      mentionText: '',
+    },
+    {
+      text: 'A multi-line\nmention with\nnewlines @mention',
+      textPrefix: 'A multi-line\nmention with\nnewlines ',
+      mentionText: 'mention',
+    },
+    {
+      text: 'text @first mention @second mention',
+      textPrefix: 'text @first mention ',
+      mentionText: 'second mention',
+    },
+    {
+      text: 'text text \n@first \n@second mention',
+      textPrefix: 'text text \n@first \n',
+      mentionText: 'second mention',
+    },
+  ];
+
+  it('should match valid mentions', () =>
+    validMatches.forEach(validMatchObject => {
+      const match = validMatchObject.text.match(webMentionTypeaheadRegex);
+      expect(match).toBeTruthy();
+      expect(match?.groups?.textPrefix ?? '').toEqual(
+        validMatchObject.textPrefix,
+      );
+      expect(match?.groups?.mentionText ?? '').toEqual(
+        validMatchObject.mentionText,
+      );
+    }));
+
+  const invalidMatches = [
+    'This is not a valid text with mention',
+    `@${'a'.repeat(200)} `,
+    'text prefix\\@mention',
+    '@mention\nnewline \\@mention with space',
+    '\\@',
+    '\\@username',
+    '\\@thread with spaces',
+  ];
+
+  it('should not match invalid mentions', () =>
+    invalidMatches.forEach(text =>
+      expect(text.match(webMentionTypeaheadRegex)).toBeFalsy(),
+    ));
+});