add_regex returns inspected value for non Regexp objects. Workaround, so users don't have to care about enclosing values in double quotes.

actionpack/lib/action_dispatch/testing/assertions/selector.rb

@@ -428,7 +428,8 @@ def initialize(substitute = '?')
             end
 
             def add_regex(regex)
-              return regex unless regex.is_a?(Regexp)
+              # Nokogiri doesn't like arbitrary values without quotes, hence inspect.
+              return regex.inspect unless regex.is_a?(Regexp)
               @regexes.push(regex)
               last_id.to_s # avoid implicit conversions of Fixnum to String
             end