To expand upon Kirk Larkin's answer: The "dataAnnotations:dataTypeAttribute:disableRegEx" app setting is only available since .NET Framework 4.6.1, according to dotnet pull request #668 (re-add ASPNET472CompatDoc).
I also checked by downloading the .NET Framework 4.6 (58 MB .zip) and 4.6.1 RTM (59 MB .zip) source and running the following git diff command.
git diff D:/src/dotnet46/Source/ndp/fx/src/xsp/system/DataAnnotations/DataAnnotations/UrlAttribute.cs D:/src/dotnet461RTM/Source/ndp/fx/src/xsp/system/DataAnnotations/DataAnnotations/UrlAttribute.cs
Indeed, the new-as-of-4.6.1 internal static class AppSettings is what reads the new-as-of-4.6.1 app setting.
And ... My point in adding this answer is that I am out of luck on net452.