diff --git a/public/include/classes/user.class.php b/public/include/classes/user.class.php
index cd514966..a3a01ef5 100644
--- a/public/include/classes/user.class.php
+++ b/public/include/classes/user.class.php
@@ -455,7 +455,7 @@ class User {
       $this->setErrorMessage('Username exceeding character limit');
       return false;
     }
-    if (!preg_match('/[^a-zA-Z0-9_\-]/', $username)) {
+    if (preg_match('/[^a-zA-Z0-9_\-]/', $username)) {
       $this->setErrorMessage('Username may only contain alphanumeric characters');
       return false;
     }