Ported missing functionality from Rails 2.3.x, raise error on wrong regexps in :constraints in routes

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -113,6 +113,15 @@ module ActionDispatch
             @requirements ||= (@options[:constraints].is_a?(Hash) ? @options[:constraints] : {}).tap do |requirements|
               requirements.reverse_merge!(@scope[:constraints]) if @scope[:constraints]
               @options.each { |k, v| requirements[k] = v if v.is_a?(Regexp) }
+
+              requirements.each do |_, requirement|
+                if requirement.source =~ %r{\A(\\A|\^)|(\\Z|\\z|\$)\Z}
+                  raise ArgumentError, "Regexp anchor characters are not allowed in routing requirements: #{requirement.inspect}"
+                end
+                if requirement.multiline?
+                  raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"
+                end
+              end
             end
           end