using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string foo = "<mama id=\"priceblock_ourprice\" class=\"a-size-medium a-color-price priceBlockBuyingPriceString\">CDN$&nbsp;23.99</mama>";

        Regex rx = new Regex(@"<mama .*>(.*)<\/mama>", RegexOptions.Compiled | RegexOptions.IgnoreCase);

        MatchCollection matches = rx.Matches(foo);

        Console.WriteLine(matches[0].Groups[1].Value);

    }

}