Index: wp-includes/functions-formatting.php
===================================================================
--- wp-includes/functions-formatting.php	(revision 3781)
+++ wp-includes/functions-formatting.php	(working copy)
@@ -278,7 +278,7 @@
 
 	// If strict, reduce to ASCII for max portability.
 	if ( $strict )
-		$username = preg_replace('|[^a-z0-9 _.-@]|i', '', $username);
+		$username = preg_replace('|[^a-z0-9 _.\-@]|i', '', $username);
 
 	return apply_filters('sanitize_user', $username, $raw_username, $strict);
 }