Can't reference Regex in string extension method

I have run into very strange problem: I have made extension method for string like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Vaniv.Mvc
{
    public static class StringHelpers
    {
        public static string ToSeoUrl(this string url)
        {
            // make the url lowercase
            string encodedUrl = (url ?? "").ToLower();
            System.Text.RegularExpressions.Regex rgx = new System.Text.RegularExpre开发者_开发问答ssions.Regex("[^a-zA-Z0-9 -]");
            encodedUrl = rgx.Replace(encodedUrl, "-");

            return encodedUrl;
        }
    }
}

And problem that durning runstime I get error: CS0246: The type or namespace name 'Regex' could not be found (are you missing a using directive or an assembly reference?)

Im not missing using directive. Nor do I missing Assembly (I can use Regex withing controller for example). I have put my extension method into App_Code, but doubt it have any connection,

Move the cs file to another directory (out of App_Code folder) put it on the root of the project.

Check this article