diff --git a/src/Symfony/Component/EventDispatcher/DependencyInjection/RegisterListenersPass.php b/src/Symfony/Component/EventDispatcher/DependencyInjection/RegisterListenersPass.php
index 1a52c3e865c9d..83fc73e3eb063 100644
--- a/src/Symfony/Component/EventDispatcher/DependencyInjection/RegisterListenersPass.php
+++ b/src/Symfony/Component/EventDispatcher/DependencyInjection/RegisterListenersPass.php
@@ -81,7 +81,7 @@ public function process(ContainerBuilder $container)
 
                 if (!isset($event['method'])) {
                     $event['method'] = 'on'.preg_replace_callback([
-                        '/(?<=\b)[a-z]/i',
+                        '/(?<=[\b\._])[a-z]/i',
                         '/[^a-z0-9]/i',
                     ], function ($matches) { return strtoupper($matches[0]); }, $event['event']);
                     $event['method'] = preg_replace('/[^a-z0-9]/i', '', $event['method']);