Unsupported regex flag

[flash1293]

Kibana version: master

Browser version: Firefox, IE 11

Describe the bug: Usage of an unsupported regex flag is breaking Discovery.
Seems to be introduced by 5820318#diff-3

Firefox logs
Possibly unhandled rejection: {"message":"invalid regular expression flag s","stack":"KbnError@http://localhost:5701/rkz/bundles/commons.bundle.js:110363:20\n_flatten/<@http://localhost:5701/rkz/bundles/commons.bundle.js:103584:19\nprocessQueue@webpack://%5Bname%5D/./node_modules/angular/angular.js?:17169:37\nscheduleProcessQueue/<@webpack://%5Bname%5D/./node_modules/angular/angular.js?:17217:27\n$digest@webpack://%5Bname%5D/./node_modules/angular/angular.js?:18352:15\n$evalAsync/<@webpack://%5Bname%5D/./node_modules/angular/angular.js?:18581:15\ncompleteOutstandingRequest@webpack://%5Bname%5D/./node_modules/angular/angular.js?:6428:7\nBrowser/self.defer/timeoutId<@webpack://%5Bname%5D/./node_modules/angular/angular.js?:6707:7\n"}

Steps to reproduce:

1. Go to Discovery Tab

[elasticmachine]

Pinging @elastic/kibana-app