How to verify that strings are in valid email format

The following example uses a regular expression to verify 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.

Pattern Description
(@) Match the @ character. This is the first capturing group.
(.+) Match one or more occurrences of any character. This is the second capturing group.
$ End the match at the end of the string.

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. In addition, the IsValidEmail method does not verify that the top-level domain name is a valid domain name listed at the IANA Root Zone Database, which would require a look-up operation. Instead, the regular expression merely verifies that the top-level domain name consists of between two and twenty-four ASCII characters, with alphanumeric first and last characters and the remaining characters being either alphanumeric or a hyphen (-).

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

public class RegexUtilities
{
    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));

            // Examines the domain part of the email and normalizes it.
            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;
            }
        }
        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;
        }
    }
}
Imports System.Globalization
Imports System.Text.RegularExpressions

Public Class RegexUtilities

    Public Shared Function IsValidEmail(email As String) As Boolean

        If String.IsNullOrWhiteSpace(email) Then Return False

        ' Use IdnMapping class to convert Unicode domain names.
        Try
            'Examines the domain part of the email and normalizes it.
            Dim DomainMapper =
                Function(match As Match) As String

                    'Use IdnMapping class to convert Unicode domain names.
                    Dim idn = New IdnMapping

                    'Pull out and process domain name (throws ArgumentException on invalid)
                    Dim domainName As String = idn.GetAscii(match.Groups(2).Value)

                    Return match.Groups(1).Value & domainName
                    
                End Function

            'Normalize the domain
            email = Regex.Replace(email, "(@)(.+)$", DomainMapper,
                                  RegexOptions.None, TimeSpan.FromMilliseconds(200))

        Catch e As RegexMatchTimeoutException
            Return False

        Catch e As ArgumentException
            Return False

        End Try

        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 e As RegexMatchTimeoutException
            Return False

        End Try

    End Function

End Class

In this example, the regular expression pattern ^(?(")(".+?(?<!\\)"@)|(([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]))$ is interpreted as shown in the following legend. The regular expression is compiled using the RegexOptions.IgnoreCase flag.

Pattern ^: Begin the match at the start of the string.

Pattern (?("): Determine whether the first character is a quotation mark. (?(") is the beginning of an alternation construct.

Pattern (?(")(".+?(?<!\\)"@): If the first character is a quotation mark, match a beginning quotation mark followed by at least one occurrence of any character, followed by an ending quotation mark. The ending quotation mark must not be preceded by a backslash character (\). (?<! is the beginning of a zero-width negative lookbehind assertion. The string should conclude with an at sign (@).

Pattern |(([0-9a-z]: If the first character is not a quotation mark, match any alphabetic character from a to z or A to Z (the comparison is case insensitive), or any numeric character from 0 to 9.

Pattern (\.(?!\.)): If the next character is a period, match it. If it is not a period, look ahead to the next character and continue the match. (?!\.) is a zero-width negative lookahead assertion that prevents two consecutive periods from appearing in the local part of an email address.

Pattern |[-!#\$%&'\*\+/=\?\^`\{\}\|~\w]: If the next character is not a period, match any word character or one of the following characters: -!#$%&'*+/=?^`{}|~

Pattern ((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*: Match the alternation pattern (a period followed by a non-period, or one of a number of characters) zero or more times.

Pattern @: Match the @ character.

Pattern (?<=[0-9a-z]): Continue the match if the character that precedes the @ character is A through Z, a through z, or 0 through 9. This pattern defines a zero-width positive lookbehind assertion.

Pattern (?(\[): Check whether the character that follows @ is an opening bracket.

Pattern (\[(\d{1,3}\.){3}\d{1,3}\]): If it is an opening bracket, match the opening bracket followed by an IP address (four sets of one to three digits, with each set separated by a period) and a closing bracket.

Pattern |(([0-9a-z][-0-9a-z]*[0-9a-z]*\.)+: If the character that follows @ is not an opening bracket, match one alphanumeric character with a value of A-Z, a-z, or 0-9, followed by zero or more occurrences of a hyphen, followed by zero or one alphanumeric character with a value of A-Z, a-z, or 0-9, followed by a period. This pattern can be repeated one or more times, and must be followed by the top-level domain name.

Pattern [a-z0-9][\-a-z0-9]{0,22}[a-z0-9])): The top-level domain name must begin and end with an alphanumeric character (a-z, A-Z, and 0-9). It can also include from zero to 22 ASCII characters that are either alphanumeric or hyphens.

Pattern $: End the match at the end of the string.

Compile 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.

You can also use the Regex.CompileToAssembly method to include this regular expression in a regular expression library.

If they are used in a regular expression library, you can call them by using code such as the following:

class Program
{
    static void Main(string[] args)
    {
        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", "j.s@server1.proseware.com",
                                    "\"j\\\"s\\\"\"@proseware.com", "js@contoso.中国" };

        foreach (var emailAddress in emailAddresses)
        {
            if (RegexUtilities.IsValidEmail(emailAddress))
                Console.WriteLine($"Valid:   {emailAddress}");
            else
                Console.WriteLine($"Invalid: {emailAddress}");
        }

        Console.ReadKey();
    }
}
// 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
//       Valid: 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
//       Valid: js@proseware.com9
//       Valid: j.s@server1.proseware.com
//       Valid: "j\"s\""@proseware.com
//       Valid: js@contoso.中国
Public Class Application
   Public Shared Sub Main()
      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", "j.s@server1.proseware.com",
                                       """j\""s\""""@proseware.com", "js@contoso.中国"}

      For Each emailAddress As String In emailAddresses
         If RegexUtilities.IsValidEmail(emailAddress) Then
               Console.WriteLine($"Valid:   {emailAddress}")
         Else
               Console.WriteLine($"Invalid: {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
'       Valid: 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
'       Valid: js@proseware.com9
'       Valid: j.s@server1.proseware.com
'       Valid: "j\"s\""@proseware.com
'       Valid: js@contoso.中国

See also