using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {
  string input = @"<p thing=""stuff; stuff"" style=""color: #000000; text-transform: none; letter-spacing: normal; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; word-spacing: 0px; white-space: normal; font-size-adjust: none; font-stretch: normal; -webkit-text-stroke-width: 0px;"">";
  string output = Regex.Replace(input, @"(?<=<p)([^>]*?)\s*style=""[^"">]+""", "$1");
  Console.WriteLine(output);
    }
}