using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

         string pattern = @"(<.+?)\s+style\s*=\s*([""']).*?\2(.*?>)";

        string substitution = @"$1$3";

        string input = @"<p style=""margin: 15px 0px; padding: 0px; border: 0px; outline: 0px;"">Hello</p>";

        Regex regex = new Regex(pattern);

        string result = regex.Replace(input, substitution);

        Console.WriteLine(result);

    }

}