How Do I Make A Web Crawler That Works Through Desktop Application, That Application Would Separate Internal And External Links

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C#

A web crawler desktop app using c# that would separate internal and external links i.e. <a href="about.html"> is internal and <a href="http|https://www.xyz.com"> is external, I've tried many solutions but all are finding links great but no solution for separation of internal and external links of a website for crawling is available.
I'm using the following code to separate internal and external links but it doesn't work as I need. It's been 2 days I'm working on it but still no improvements. Can you check this and guide me about it.

List inter = new List();
List dates = new List();
int count = 0;
List i2 = new List();
WebClient web = new WebClient();
string html = web.DownloadString(textBox1.Text);
string n3 = "", s4 = "";
MatchCollection m0 = Regex.Matches(html, @"]*?href[\s]?=[\s\""\']+(?.*?)[\""\']+.*?>(?[^<]+|.*?)?<\/a>", RegexOptions.Singleline | RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);

foreach (Match m in m0)
{
string city = m.Groups[1].Value;



Match m2 = Regex.Match(city, "\\s*(?i)href\\s*=\\s*(\"([^\"]*\")|'[^']*'|([^'\">\\s]+))", RegexOptions.Singleline | RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);
string city2 = m2.Groups[1].Value;
dates.Add(city2);

s4 = textBox1.Text;
string n2 = s4.Remove(0, 11);
n3 = s4.Remove(0, 12);
string n4 = s4.Remove(0,7);

Match m3 = Regex.Match(city, @"((www\.|(http|https|ftp|news|file)+\:\/\/)[_.a-z0-9-]+\.[a-z0-9\/_:@=.+?,##%&~-]*[^.|\'|\# |!|\(|?|,| |>|<|;|\)])", RegexOptions.Singleline | RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);
string s5 = m3.Groups[1].Value;
if (m3.Groups[1].Value != s4 && m3.Groups[1].Value != n2 && m3.Groups[1].Value != n3&& m3.Groups[1].Value!=n4)
{



i2.Add(city);
inter.Add(s5);

count = 1;

}


}
if (count != 0)
{
AllLinks.Items.Add(s4);
}
Hrefs.DataSource = i2;
//AllLinks.DataSource = dates;
inter.RemoveAll(string.IsNullOrWhiteSpace);
ExternalLinks.DataSource = inter;