Hope this will work http://world.episerver.com/documentation/developer-guides/forms/create-custom-form-element-with-validator/

Thanks Anish,

But that is a totally different path, which takes a lot of time. EpiServer Forms is already providing regex validation and I don't want to customize that, I just want to show the correct message for it. As you may know, ASP.NET data annotation syntax is like this:

[RegularExpression("^([a-zA-Z0-9 .&'-]+)$", ErrorMessage = "Invalid First Name")]
public string FirstName { get; set; }

This is a very common scenario for validation. 


Thanks again,
Sia

One of my friends at work found the solution.

Create a Forms.xml inside \Resources\LanguageFiles and add this as the content:

<?xml version="1.0" encoding="utf-8" ?>
<languages>
  <language name="English" id="en">
    <episerver>
      <forms>
        <validators>
          <episerver.forms.implementation.validation.regularexpressionvalidator>
            <displayname>Regular expression</displayname>
            <message>This field should be in the correct format.</message>
          </episerver.forms.implementation.validation.regularexpressionvalidator>
        </validators>
      </forms>
    </episerver>
  </language>
</languages>

That's not a very ideal solution, because it's a general message for all regex, but still works!

Thanks for the above workaround, this has been an issue for us too.