diff --git a/lib/liquid/lexer.rb b/lib/liquid/lexer.rb
index d8d8b566..f33e13e5 100644
--- a/lib/liquid/lexer.rb
+++ b/lib/liquid/lexer.rb
@@ -29,7 +29,7 @@ module Liquid
       '[' => :open_square,
       ']' => :close_square
     }
-    IDENTIFIER = /[\w\-?]+/
+    IDENTIFIER = /[\w\-?!]+/
     SINGLE_STRING_LITERAL = /'[^\']*'/
     DOUBLE_STRING_LITERAL = /"[^\"]*"/
     INTEGER_LITERAL = /-?\d+/