using System;
using System.Data;
using System.Text.RegularExpressions;
class Test
{

    static void Main()
    {
        string strHtml = "你得到的HTML";
        MatchCollection mc = Regex.Matches("<table>[\\s\\S].*?</table>");
        foreach (Match m in mc)
        {
            Console.WriteLine(m.Value);
        }
    }
}

using System;
using System.Data;
using System.Text.RegularExpressions;
class Test
{
    static void Main()
    {
        string strHtml = "你得到的HTML";
        MatchCollection mc = Regex.Matches("<table.*?>[\\s\\S].*?</table.*?>");//正则修改了下！这样好一些！
        foreach (Match m in mc)
        {
            Console.WriteLine(m.Value);
        }
    }
}

<table[^>]+?>[\s\S]*?\</table>

using System;
using System.Data;
using System.Text.RegularExpressions;
class Test
{

    static void Main()
    {
        string strHtml = "你得到的HTML";
        MatchCollection mc = Regex.Matches(strHtml,"<table.*?>[\\s\\S].*?</table.*?>");
        foreach (Match m in mc)
        {
            Console.WriteLine(m.Value);
        }
    }
}

 Dim sRemote As String = "^\<table width\=\""99\%\"" align\=\""center\"" id\=\""main\-content\"" cellpadding\=\""2\"" cellspacing\=\""0""\>[\\s\\S].*?\<\/td\>\<\/tr\>\<\/table\>$"
        Dim strHtml As String = GetRequestString(spath, 5000, 0, UTF8Encoding.UTF8)
    
        strHtml = GetRegValue(strHtml, sRemote)
        Me.Label1.Text = strHtml

    Public Function GetRegValue(ByVal RegexString As String, ByVal RemoteStr As String)
        Dim MatchVale As String = ""
        Dim r As Regex = New Regex(RegexString)
        Dim m As Match = r.Match(RemoteStr)

        If m.Success Then
            MatchVale = m.Value
        End If

        Return MatchVale
    End Function

string tt = "<table width=\"99%\" align=\"center\" id=\"main-content\" cellpadding=\"2\" cellspacing=\"0\"><tr><td><table><tr><td></td></tr></table></td></tr></table>";
 
string reg = @"<table[^>]*>(?<sss>[\s\S]*)</td></tr></table>";


Match m = Regex.Match(tt, reg, RegexOptions.IgnoreCase);

string result = m.Groups["sss"].ToString();