Index: /elgg/trunk/lib/elgglib.php
===================================================================
--- /elgg/trunk/lib/elgglib.php (revision 37)
+++ /elgg/trunk/lib/elgglib.php (revision 150)
@@ -2055,5 +2055,5 @@
 
     // TODO: modifying regex needs update rewrite rules
-    $regex = sprintf('/^[A-Za-z0-9]{%d,%d}$/i', $minchars, $maxchars); // letters and numbers only
+    $regex = sprintf('/^[A-Za-z0-9_]{%d,%d}$/i', $minchars, $maxchars); // letters and numbers only
 
     // TODO: should allow plugins to extend validation?