diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index d370d75..fd95d76 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -488,7 +488,7 @@
 		if ( ! in_array('required', $rules) AND is_null($postdata))
 		{
 			// Before we bail out, does the rule contain a callback?
-			if (preg_match("/(callback_\w+)/", implode(' ', $rules), $match))
+			if (preg_match("/(callback_\w+(\[.*?\])?)/", implode(' ', $rules), $match))
 			{
 				$callback = TRUE;
 				$rules = (array('1' => $match[1]));