Add note on RegularExpression attribute validation (dotnet#15984)

* Add note on RegularExpression attribute validation

This note will help inform developers of the limitations.

* Update validation.md

* Update validation.md

* Update validation.md

* Update validation.md

* Update validation.md

* Update validation.md

* Update validation.md

Author: jasonleenaylor

aspnetcore/mvc/models/validation.md

@@ -4,7 +4,7 @@ author: rick-anderson
 description: Learn about model validation in ASP.NET Core MVC and Razor Pages.
 ms.author: riande
 ms.custom: mvc
-ms.date: 12/05/2019
+ms.date: 12/15/2019
 uid: mvc/models/validation
 ---
 
@@ -419,6 +419,8 @@ Built-in validation attributes include:
 * `[Url]`: Validates that the property has a URL format.
 * `[Remote]`: Validates input on the client by calling an action method on the server. See [`[Remote]` attribute](#remote-attribute) for details about this attribute's behavior.
 
+When using the `[RegularExpression]` attribute with client-side validation, the regex is executed in JavaScript on the client. This means [ECMAScript](/dotnet/standard/base-types/regular-expression-options#ecmascript-matching-behavior) matching behavior will be used. For more information, see [this GitHub issue](https://github.com/dotnet/corefx/issues/42487).
+
 A complete list of validation attributes can be found in the [System.ComponentModel.DataAnnotations](xref:System.ComponentModel.DataAnnotations) namespace.
 
 ### Error messages