Make sure to expose the regex during install so the validation actually works

src/Umbraco.Web.UI/umbraco/Install/Views/Index.cshtml

@@ -1,4 +1,5 @@
-@using Umbraco.Web
+@using Umbraco.Core.Configuration
+@using Umbraco.Web
 @using Umbraco.Web.Install.Controllers
 @{
     Layout = null;
@@ -66,6 +67,8 @@
             "installApiBaseUrl": "@ViewBag.InstallApiBaseUrl",
             "umbracoBaseUrl": "@ViewBag.UmbracoBaseFolder"
         };
+        Umbraco.Sys.ServerVariables.umbracoSettings = {};
+        Umbraco.Sys.ServerVariables.umbracoSettings.emailRegex = "@(UmbracoConfig.For.UmbracoSettings().Content.EmailRegex)";
     </script>
     <script src="lib/rgrove-lazyload/lazyload.js"></script>
     <script src="js/install.loader.js"></script>