RegularExpressionAttribute - How to make it not case sensitive for client side validation?

I created this attribute which allows you to specify RegexOptions. EDIT: It also integrates with unobtrusive validation. The client will only obey RegexOptions.Multiline and RegexOptions.IgnoreCase since that is what JavaScript supports.

[RegularExpressionWithOptions(@".+@example\.com", RegexOptions = RegexOptions.IgnoreCase)]

C#

public class RegularExpressionWithOptionsAttribute : RegularExpressionAttribute, IClientValidatable
{
    public RegularExpressionWithOptionsAttribute(string pattern) : base(pattern) { }

    public RegexOptions RegexOptions { get; set; }

    public override bool IsValid(object value)
    {
        if (string.IsNullOrEmpty(value as string))
            return true;

        return Regex.IsMatch(value as string, "^" + Pattern + "$", RegexOptions);
    }

    public IEnumerable<System.Web.Mvc.ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = FormatErrorMessage(metadata.DisplayName),
            ValidationType = "regexwithoptions"
        };

        rule.ValidationParameters["pattern"] = Pattern;

        string flags = "";
        if ((RegexOptions & RegexOptions.Multiline) == RegexOptions.Multiline)
            flags += "m";
        if ((RegexOptions & RegexOptions.IgnoreCase) == RegexOptions.IgnoreCase)
            flags += "i";
        rule.ValidationParameters["flags"] = flags;

        yield return rule;
    }
}

JavaScript

(function ($) {

    $.validator.unobtrusive.adapters.add("regexwithoptions", ["pattern", "flags"], function (options) {
        options.messages['regexwithoptions'] = options.message;
        options.rules['regexwithoptions'] = options.params;
    });

    $.validator.addMethod("regexwithoptions", function (value, element, params) {
        var match;
        if (this.optional(element)) {
            return true;
        }

        var reg = new RegExp(params.pattern, params.flags);
        match = reg.exec(value);
        return (match && (match.index === 0) && (match[0].length === value.length));
    });

})(jQuery);

This article by Anthony Stevens helped me get this working: ASP.NET MVC 3 Unobtrusive Javascript Validation With Custom Validators

In C# you can inline some regex options. To specify the option to ignore case you would add (?i) to the beginning of your pattern. However, I am not sure how this would be treated by the RegularExpressionAttribute and if it handles translation for client-side. From my experience with ASP.NET's RegularExpressionValidator I doubt it; the regex should be vanilla enough to work for both engines.

In any case if it was valid it would look like this:

@"^(?i)(?:\b(?:\d{5}(?:\s*-\s*\d{5})?|([A-Z]{2})\d{3}(?:\s*-\s*\1\d{3})?)(?:,\s*)?)+$"

private const String regex = @"^(?:\b(?:\d{5}(?:\s*-\s*\d{5})?|([a-zA-Z]{2})\d{3}(?:\s*-\s*\1\d{3})?)(?:,\s*)?)+$";