using System;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var s = "<Element><Element value=\"'hello&stack<overflow>'\" value=\"'hi&stack<over flow2 >'\"/></Element>";
		var rx = @"((?:<[a-zA-Z][\w:-]*|\G(?!\A))\s+[^\s=<]*=)(""[^""]*"")";
		var clean = Regex.Replace(s, rx, m => 
			string.Format("{0}{1}", m.Groups[1].Value, m.Groups[2].Value.Replace("&", "&amp;").Replace("<", "&lt;").Replace(">", "&gt;"))
		);
		Console.Write(clean);
	}
}