diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index d7905b581fd..89e3a9ea00d 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -380,7 +380,7 @@ function GETPOST($paramname, $check='', $method=0, $filter=NULL, $options=NULL)
 	            break;
 	        case 'aZ09':
 	            $out=trim($out);
-	            if (preg_match('/[^a-z0-9]+/i',$out)) $out='';
+	            if (preg_match('/[^a-z0-9_]+/i',$out)) $out='';
 	            break;
 	        case 'array':
 	            if (! is_array($out) || empty($out)) $out=array();
diff --git a/htdocs/index.php b/htdocs/index.php
index 460082399ba..1431f73ca7d 100644
--- a/htdocs/index.php
+++ b/htdocs/index.php
@@ -30,7 +30,7 @@ require 'main.inc.php';
 require_once DOL_DOCUMENT_ROOT.'/core/class/html.formother.class.php';
 
 // If not defined, we select menu "home"
-$_GET['mainmenu']=GETPOST('mainmenu', 'alpha')?GETPOST('mainmenu', 'alpha'):'home';
+$_GET['mainmenu']=GETPOST('mainmenu', 'aZ09')?GETPOST('mainmenu', 'aZ09'):'home';
 $action=GETPOST('action');
 
 $hookmanager->initHooks(array('index'));