How to: Verify That Strings Are in Valid E-Mail Format

Updated: December 2011

The following example verifies that a string is in valid email format.

Example

The example defines an IsValidEmail method, which returns true if the string contains a valid email address and false if it does not, but takes no other action.

To verify that the email address is valid, the IsValidEmail method calls the Regex.Replace(String, String, MatchEvaluator) method with the (@)(.+)$ regular expression pattern to separate the domain name from the email address. The third parameter is a MatchEvaluator delegate that represents the method that processes and replaces the matched text. The regular expression pattern is interpreted as follows.

The domain name along with the @ character is passed to the DomainMapper method, which uses the IdnMapping class to translate Unicode characters that are outside the US-ASCII character range to Punycode. The method also sets the invalid flag to True if the IdnMapping.GetAscii method detects any invalid characters in the domain name. The method returns the Punycode domain name preceded by the @ symbol to the IsValidEmail method.

The IsValidEmail method then calls the Regex.IsMatch(String, String) method to verify that the address conforms to a regular expression pattern.

Note that the IsValidEmail method does not perform authentication to validate the email address. It merely determines whether its format is valid for an email address.

Imports System.Globalization
Imports System.Text.RegularExpressions

Public Class RegexUtilities
   Dim invalid As Boolean = False

   public Function IsValidEmail(strIn As String) As Boolean
       invalid = False
       If String.IsNullOrEmpty(strIn) Then Return False

       ' Use IdnMapping class to convert Unicode domain names.
       strIn = Regex.Replace(strIn, "(@)(.+)$", AddressOf Me.DomainMapper)
       If invalid Then Return False

       ' Return true if strIn is in valid e-mail format.
       Return Regex.IsMatch(strIn, _
              "^(?("")(""[^""]+?""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" + _
              "(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9]{2,17}))$", 
              RegexOptions.IgnoreCase)
   End Function

   Private Function DomainMapper(match As Match) As String
      ' IdnMapping class with default property values.
      Dim idn As New IdnMapping()

      Dim domainName As String = match.Groups(2).Value
      Try
         domainName = idn.GetAscii(domainName)
      Catch e As ArgumentException
         invalid = True      
      End Try      
      Return match.Groups(1).Value + domainName
   End Function
End Class

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

public class RegexUtilities
{
   bool invalid = false;

   public bool IsValidEmail(string strIn)
   {
       invalid = false;
       if (String.IsNullOrEmpty(strIn))
          return false;

       // Use IdnMapping class to convert Unicode domain names.
       strIn = Regex.Replace(strIn, @"(@)(.+)$", this.DomainMapper);
       if (invalid) 
          return false;

       // Return true if strIn is in valid e-mail format.
       return Regex.IsMatch(strIn, 
              @"^(?("")(""[^""]+?""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" + 
              @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9]{2,17}))$", 
              RegexOptions.IgnoreCase);
   }

   private string DomainMapper(Match match)
   {
      // IdnMapping class with default property values.
      IdnMapping idn = new IdnMapping();

      string domainName = match.Groups[2].Value;
      try {
         domainName = idn.GetAscii(domainName);
      }
      catch (ArgumentException) {
         invalid = true;      
      }      
      return match.Groups[1].Value + domainName;
   }
}

In this example, the regular expression pattern ^(?("")(""[^""]+?""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9]{2,17}))$ is interpreted as shown in the following table. Note that the regular expression is compiled using the RegexOptions.IgnoreCase flag.

Compiling the Code

The IsValidEmail and DomainMapper methods can be included in a library of regular expression utility methods, or they can be included as private static or instance methods in the application class. If they are used in a regular expression library, you can call them by using code such as the following:

Public Class Application
   Public Shared Sub Main()
      Dim util As New RegexUtilities()
      Dim emailAddresses() As String = { "david.jones@proseware.com", "d.j@server1.proseware.com", _
                                         "jones@ms1.proseware.com", "j.@server1.proseware.com", _
                                         "j@proseware.com9", "js#internal@proseware.com", _
                                         "j_9@[129.126.118.1]", "j..s@proseware.com", _
                                         "js*@proseware.com", "js@proseware..com", _
                                         "js@proseware.com9", "js@proseware.com9", "j.s@server1.proseware.com" }

      For Each emailAddress As String In emailAddresses
         If util.IsValidEmail(emailAddress) Then
            Console.WriteLine("Valid: {0}", emailAddress)
         Else
            Console.WriteLine("Invalid: {0}", emailAddress)
         End If      
      Next                                            
   End Sub
End Class
' The example displays the following output:
'       Valid: david.jones@proseware.com
'       Valid: d.j@server1.proseware.com
'       Valid: jones@ms1.proseware.com
'       Invalid: j.@server1.proseware.com
'       Invalid: j@proseware.com9
'       Valid: js#internal@proseware.com
'       Valid: j_9@[129.126.118.1]
'       Invalid: j..s@proseware.com
'       Invalid: js*@proseware.com
'       Invalid: js@proseware..com
'       Invalid: js@proseware.com9
'       Valid: j.s@server1.proseware.com

public class Application
{
   public static void Main()
   {
      RegexUtilities util = new RegexUtilities();
      string[] emailAddresses = { "david.jones@proseware.com", "d.j@server1.proseware.com", 
                                  "jones@ms1.proseware.com", "j.@server1.proseware.com", 
                                  "j@proseware.com9", "js#internal@proseware.com", 
                                  "j_9@[129.126.118.1]", "j..s@proseware.com", 
                                  "js*@proseware.com", "js@proseware..com", 
                                  "js@proseware.com9", "js@proseware.com9", "j.s@server1.proseware.com" };

      foreach (var emailAddress in emailAddresses) {
         if (util.IsValidEmail(emailAddress))
            Console.WriteLine("Valid: {0}", emailAddress);
         else
            Console.WriteLine("Invalid: {0}", emailAddress);
      }                                            
   }
}
// The example displays the following output:
//       Valid: david.jones@proseware.com
//       Valid: d.j@server1.proseware.com
//       Valid: jones@ms1.proseware.com
//       Invalid: j.@server1.proseware.com
//       Invalid: j@proseware.com9
//       Valid: js#internal@proseware.com
//       Valid: j_9@[129.126.118.1]
//       Invalid: j..s@proseware.com
//       Invalid: js*@proseware.com
//       Invalid: js@proseware..com
//       Invalid: js@proseware.com9
//       Valid: j.s@server1.proseware.com

See Also

Other Resources

.NET Framework Regular Expressions

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