diff --git a/telebot/__init__.py b/telebot/__init__.py
index 248e9ef..3b70be9 100644
--- a/telebot/__init__.py
+++ b/telebot/__init__.py
@@ -852,7 +852,7 @@ class TeleBot:
     def _test_filter(self, filter, filter_value, message):
         test_cases = {
             'content_types': lambda msg: msg.content_type in filter_value,
-            'regexp': lambda msg: msg.content_type == 'text' and re.search(filter_value, msg.text),
+            'regexp': lambda msg: msg.content_type == 'text' and re.search(filter_value, msg.text, re.IGNORECASE),
             'commands': lambda msg: msg.content_type == 'text' and util.extract_command(msg.text) in filter_value,
             'func': lambda msg: filter_value(msg)
         }