void Main()
{
    string s="aaaa<table>bbbb<table><tersfjs>fklsk></table></table><table>cccccc<tr>sf</tr></table>fwefw";
    Regex reg=new Regex(@"(?is)<table[^>]*>(?><table[^>]*>(?<o>)|</table>(?<-o>)|(?:(?!</?table\b).)*)*(?(o)(?!))</table>");
    foreach(Match m in reg.Matches(s))
    {
     Console.WriteLine(m.Value);
    }
    /*
    <table>bbbb<table><tersfjs>fklsk></table></table>
    <table>cccccc<tr>sf</tr></table>
    */
}