diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index ee69d5cb707..42ed7df0d07 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -949,7 +949,7 @@ function clean_filename($string) {
 /// Cleans a given filename by removing suspicious or troublesome characters
     $string = stripslashes($string);
     $string = eregi_replace("\.\.", "", $string);
-    $string = eregi_replace("[^([:alnum:]|\.)]", "_", $string);
+    $string = eregi_replace("[^(-|[:alnum:]|\.)]", "_", $string);
     return    eregi_replace("_+", "_", $string);
 }