Custom RegularExpressionAttribute missing data-val-regex-pattern for client-side validation

Question

I have created the following custom RegularExpressionAttribute

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
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Answer 1

Here's another approach from How to create custom validation attribute for MVC, which is adapted from this article on ASP.NET MVC Custom Validation:

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false)]
public class AlphaNumericAttribute: RegularExpressionAttribute
{
    private const string pattern = "^[-A-Za-z0-9]+$";

    public AlphaNumericAttribute() : base(pattern)
    {
        // necessary to enable client side validation
        DataAnnotationsModelValidatorProvider.RegisterAdapter(
            typeof(AlphaNumericAttribute), 
            typeof(RegularExpressionAttributeAdapter));
    }
}

By using RegisterAdapter, you can leverage the integrations that already exist for regular expressions for your own inherited type.