diff --git a/lib/ace/mode/ruby_highlight_rules.js b/lib/ace/mode/ruby_highlight_rules.js
index 9270950d..cc2464ef 100644
--- a/lib/ace/mode/ruby_highlight_rules.js
+++ b/lib/ace/mode/ruby_highlight_rules.js
@@ -90,7 +90,7 @@ var RubyHighlightRules = function() {
 
     var builtinVariables = lang.arrayToMap(
         ("\$DEBUG|\$defout|\$FILENAME|\$LOAD_PATH|\$SAFE|\$stdin|\$stdout|\$stderr|\$VERBOSE|" +
-        "root_url|flash|session|cookies|params|request|response|logger").split("|")
+        "$!|root_url|flash|session|cookies|params|request|response|logger").split("|")
     );
 
     // regexp must not have capturing parentheses. Use (?:) instead.