MatchCollection matches = Regex.Matches(content, @"<a[^>]*href=([""'])?(?<href>[^'""]+)\1[^>]*>", RegexOptions.IgnoreCase);
foreach (Match item in matches)
{
  var href= item.Groups[1].Value;
}

IP2Location for.net Standard 组件是一个基于 .net Standard 框架的软件开发组件和数据解决方案，它使您能够实时发现您的web访问者通过ip地址来自何处。然后您可以根据访问者的国家、地区、城市、纬度、经度、邮政编码、ISP、域名、时区、连接速度、IDD代码、区号、气象站代码、气象站名称、MCC、MNC、移动品牌名称、海拔和使用类型动态调整网站内容。

文件下载：

IP2Location 组件: https://down.24kplus.com/dev/IP2Location-1.0.0.0.zip

免费 IP2Location BIN 数据： https://lite.ip2location.com

商业 IP2Location BIN 数据：  https://www.ip2location.com/database/ip2location

要求

Microsoft .NET 4.0 framework 或更高 / Microsoft .NET Core 2.0 或更高。

方法

属性

示例

static void Main(string[] args)
{
	using (Component component = new Component())
	{
		component.IPDatabasePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "IP2LOCATION.BIN");
		IPResult result = component.IPQuery("58.220.95.56");
		Console.WriteLine("AreaCode: " + result.AreaCode);
		Console.WriteLine("CountryCode: " + result.CountryCode);
		Console.WriteLine("Country: " + result.Country);
		Console.WriteLine("Region: " + result.Region);
		Console.WriteLine("City: " + result.City);
		Console.WriteLine("Latitude: " + result.Latitude);
		Console.WriteLine("Longitude: " + result.Longitude);
		...
	}
	Console.ReadKey();
}

验证字符串是否为有效的电子邮件格式

示例

该示例定义 IsValidEmail 方法，如果字符串包含有效的电子邮件地址，则该方法返回 true ，否则返回 false ，但不采取其他任何操作。

请注意， IsValidEmail 方法不执行身份验证来验证电子邮件地址。 它只确定其格式对于电子邮件地址是否有效。 此外， IsValidEmail 方法不对顶级域名是否是 IANA 根区域数据库中列出的有效域名进行验证，这需执行查找操作。 相反，正则表达式仅验证由二到二十四个 ASCII 字符组成的顶级域名，该域名以字母数字开头并以字符结尾，且其余的字符是字母数字或连字符 (-)。

using System;
using System.Globalization;
using System.Text.RegularExpressions;

public class RegexUtil
{
    public static bool IsValidEmail(string email)
    {
        if (string.IsNullOrWhiteSpace(email))
            return false;

        try
        {
            // Normalize the domain
            email = Regex.Replace(email, @"(@)(.+)$", DomainMapper,
                                  RegexOptions.None, TimeSpan.FromMilliseconds(200));
        }
        catch (RegexMatchTimeoutException e)
        {
            return false;
        }
        catch (ArgumentException e)
        {
            return false;
        }

        try
        {
            return Regex.IsMatch(email,
                @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
                @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-0-9a-z]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$",
                RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250));
        }
        catch (RegexMatchTimeoutException)
        {
            return false;
        }
    }
    static string DomainMapper(Match match)
    {
        // Use IdnMapping class to convert Unicode domain names.
        var idn = new IdnMapping();

        // Pull out and process domain name (throws ArgumentException on invalid)
        var domainName = idn.GetAscii(match.Groups[2].Value);

        return match.Groups[1].Value + domainName;
    }
}

该组件允许用户通过IP地址查询是否VPN代理、开放代理、Web代理、Tor出口、数据中心、Web主机（DCH）范围和搜索引擎机器人（SES），组件从 ip2proxy bin 数据文件中查询代理IP信息。

文件下载：

IP2Proxy 组件： https://down.24kplus.com/dev/ip2proxy-1.0.0.0.zip

免费 IP2Proxy BIN 数据： https://lite.ip2location.com

商业 IP2Proxy BIN 数据： https://www.ip2location.com/database/ip2proxy

要求

Microsoft .NET 4.0 framework 或更高 / Microsoft .NET Core 2.0 或更高.

方法

属性

示例

static void Main(string[] args)
{
	using (var component = new IP2Proxy.Component())
	{
		component.ProxyDatabasePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "IP2PROXY-PX8.BIN");
		component.UseMemoryMappedFile = true;
		var proxy = component.ProxyQuery("58.220.95.56");

		Console.WriteLine("IsProxy: " + proxy.IsProxy.ToString());
		Console.WriteLine("ProxyType: " + proxy.ProxyType);
		Console.WriteLine("CountryCode: " + proxy.CountryCode);
		Console.WriteLine("Country: " + proxy.Country);
		Console.WriteLine("Region: " + proxy.Region);
		Console.WriteLine("City: " + proxy.City);
		Console.WriteLine("ISP: " + proxy.ISP);
		Console.WriteLine("Domain: " + proxy.Domain);
		Console.WriteLine("UsageType: " + proxy.UsageType);
		Console.WriteLine("ASN: " + proxy.ASN);
		Console.WriteLine("AS: " + proxy.AS);
		Console.WriteLine("LastSeen: " + proxy.LastSeen);
	}
	Console.ReadKey();
}

Whois 简单来说，就是一个用来查询域名是否已经被注册，以及注册域名的详细信息的数据库（如域名所有人、域名注册商、域名注册日期和过期日期等）。通过域名Whois服务器查询，可以查询域名归属者联系方式，以及注册和到期时间。

以下为 C# 域名 Whois 查询代码

public static string Whois(string domain)
{
	if (domain == null)
		throw new ArgumentNullException();
	int ccStart = domain.LastIndexOf(".");
	if (ccStart < 0 || ccStart == domain.Length)
		throw new ArgumentException();
	string ret = "";
	Socket s = null;
	try
	{
		string cc = domain.Substring(ccStart + 1);
		s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
		s.Connect(new IPEndPoint(Dns.GetHostEntry(cc + ".whois-servers.net").AddressList[0], 43));
		s.Send(Encoding.ASCII.GetBytes(domain + "\r\n"));
		byte[] buffer = new byte[1024];
		int recv = s.Receive(buffer);
		while (recv > 0)
		{
			ret += Encoding.ASCII.GetString(buffer, 0, recv);
			recv = s.Receive(buffer);
		}
		s.Shutdown(SocketShutdown.Both);
	}
	catch
	{
		throw new SocketException();
	}
	finally
	{
		if (s != null)
			s.Close();
	}
	return ret;
}

调用事例：

[TestMethod]
public void TestWhoisMethod()
{
	string whois = Whois("24kplus.com");
	Debug.WriteLine(whois);
	Console.ReadKey();
}

结果：

   Server Name: 24KPLUS.COM
   IP Address: 34.92.152.179
   Registrar: DNSPod, Inc.
   Registrar WHOIS Server: whois.dnspod.com
   Registrar URL: http://www.dnspod.cn

   Domain Name: 24KPLUS.COM
   Registry Domain ID: 2377863045_DOMAIN_COM-VRSN
   Registrar WHOIS Server: whois.dnspod.com
   Registrar URL: http://www.dnspod.cn
   Updated Date: 2019-07-05T12:32:38Z
   Creation Date: 2019-04-08T03:16:41Z
   Registry Expiry Date: 2023-04-08T03:16:41Z
   Registrar: DNSPod, Inc.
   Registrar IANA ID: 1697
   Registrar Abuse Contact Email: abuse@dnspod.com
   Registrar Abuse Contact Phone: +86.95716
   Domain Status: clientTransferProhibited https://icann.org/epp#clientTransferProhibited
   Name Server: F1G1NS1.DNSPOD.NET
   Name Server: F1G1NS2.DNSPOD.NET
   DNSSEC: unsigned
   URL of the ICANN Whois Inaccuracy Complaint Form: https://www.icann.org/wicf/
>>> Last update of whois database: 2019-08-25T11:21:50Z <<<

For more information on Whois status codes, please visit https://icann.org/epp

NOTICE: The expiration date displayed in this record is the date the
registrar's sponsorship of the domain name registration in the registry is
currently set to expire. This date does not necessarily reflect the expiration
date of the domain name registrant's agreement with the sponsoring
registrar.  Users may consult the sponsoring registrar's Whois database to
view the registrar's reported date of expiration for this registration.

TERMS OF USE: You are not authorized to access or query our Whois
database through the use of electronic processes that are high-volume and
automated except as reasonably necessary to register domain names or
modify existing registrations; the Data in VeriSign Global Registry
Services' ("VeriSign") Whois database is provided by VeriSign for
information purposes only, and to assist persons in obtaining information
about or related to a domain name registration record. VeriSign does not
guarantee its accuracy. By submitting a Whois query, you agree to abide
by the following terms of use: You agree that you may use this Data only
for lawful purposes and that under no circumstances will you use this Data
to: (1) allow, enable, or otherwise support the transmission of mass
unsolicited, commercial advertising or solicitations via e-mail, telephone,
or facsimile; or (2) enable high volume, automated, electronic processes
that apply to VeriSign (or its computer systems). The compilation,
repackaging, dissemination or other use of this Data is expressly
prohibited without the prior written consent of VeriSign. You agree not to
use electronic processes that are automated and high-volume to access or
query the Whois database except as reasonably necessary to register
domain names or modify existing registrations. VeriSign reserves the right
to restrict your access to the Whois database in its sole discretion to ensure
operational stability.  VeriSign may restrict or terminate your access to the
Whois database for failure to abide by these terms of use. VeriSign
reserves the right to modify these terms at any time.

The Registry database contains ONLY .COM, .NET, .EDU domains and
Registrars.

C# .NET 利用 HttpWebRequest 模拟浏览器 http、https 请求。

/// <summary>
/// Http 请求
/// </summary>
/// <param name="url">url地址</param>
/// <param name="data">请求参数（可选参数）</param>
/// <param name="method">请求方法（可选参数，默认：GET）</param>
/// <param name="timeout">连接超时（可选参数,默认: 100,000 毫秒（100 秒））</param>
/// <param name="encoding">编码（可选参数，默认：utf-8）</param>
/// <param name="contentType">Content-type HTTP 标头（可选参数）</param>
/// <param name="userAgent">模拟浏览器UA（可选参数）</param>
/// <param name="headers">HTTP 标头（可选参数）</param>
/// <returns>远程服务器返回的数据</returns>
public static string HttpRequest(string url, string data = null, string method = "GET", int? timeout = null, Encoding encoding = null, string contentType = "application/x-www-form-urlencoded", string userAgent = null, Dictionary<string, string> headers = null)
{
	encoding = encoding ?? Encoding.UTF8;
	var myWebRequest = (HttpWebRequest)WebRequest.Create(url);
	myWebRequest.ContentType = contentType;
	myWebRequest.Method = method;
	if (!string.IsNullOrWhiteSpace(userAgent))
	{
		myWebRequest.UserAgent = userAgent;
	}
	if (timeout.HasValue)
	{
		myWebRequest.Timeout = timeout.Value;
	}
	if (myWebRequest.RequestUri.Scheme == "https")
	{
		myWebRequest.ProtocolVersion = HttpVersion.Version11;
		myWebRequest.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CertificateValidationCallback);
		// 这里设置了协议类型。
		ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;// SecurityProtocolType.Tls1.2; 
		myWebRequest.KeepAlive = false;
		ServicePointManager.CheckCertificateRevocationList = true;
		ServicePointManager.DefaultConnectionLimit = 100;
		ServicePointManager.Expect100Continue = false;
	}
	if (headers != null && headers.Count > 0)
	{
		foreach (KeyValuePair<string, string> item in headers)
		{
			myWebRequest.Headers[item.Key] = item.Value;
		}
	}
	try
	{
		if (!string.IsNullOrWhiteSpace(data))
		{
			using (Stream stream = myWebRequest.GetRequestStream())
			{
				var bytes = encoding.GetBytes(data);
				stream.Write(bytes, 0, bytes.Length);
				stream.Flush();
			}
		}
		using (HttpWebResponse myWebResponse = (HttpWebResponse)myWebRequest.GetResponse())
		{
			if (myWebResponse.StatusCode == HttpStatusCode.OK)
			{
				using (Stream receiveStream = myWebResponse.GetResponseStream())
				{
					using (StreamReader readStream = new StreamReader(receiveStream, Encoding.UTF8))
					{
						return readStream.ReadToEnd();
					}
				}
			}
			else
			{
				throw new WebException(myWebResponse.StatusDescription);
			}
		}
	}
	catch (Exception ex)
	{
		throw ex;
	}
	finally
	{
		myWebRequest.Abort();
	}
}

/// <summary>
/// (SSL) 证书验证回调
/// </summary>
/// <param name="sender">一个对象，它包含此验证的状态信息。</param>
/// <param name="certificate">用于对远程方进行身份验证的证书。</param>
/// <param name="chain">与远程证书关联的证书颁发机构链。</param>
/// <param name="sslPolicyErrors">与远程证书关联的一个或多个错误。</param>
/// <returns>是否接受指定证书进行身份验证</returns>
private static bool CertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
	if (sslPolicyErrors == SslPolicyErrors.None)
		return true;

	Console.WriteLine("Certificate error: {0}", sslPolicyErrors);
	return false;
}

IP地址转数字

public long IPToNumber(string ip)
{
	string[] arrayIP = ip.Split('.');
	long sip1 = long.Parse(arrayIP[0]);
	long sip2 = long.Parse(arrayIP[1]);
	long sip3 = long.Parse(arrayIP[2]);
	long sip4 = long.Parse(arrayIP[3]);
	long ipNum = sip1 * 256 * 256 * 256 + sip2 * 256 * 256 + sip3 * 256 + sip4;
	return ipNum;
}

数字转IP地址

public string NumberToIP(long ipNum)
{
	StringBuilder builder = new StringBuilder();
	builder.Append((ipNum >> 24) & 0xFF).Append(".");
	builder.Append((ipNum >> 16) & 0xFF).Append(".");
	builder.Append((ipNum >> 8) & 0xFF).Append(".");
	builder.Append(ipNum & 0xFF);
	return builder.ToString();
}