Validating input for an email address

Use this pretty, simple, and pretty simple regular expression validator:

<asp:RegularExpressionValidator ID="valEmailAddress" ControlToValidate="EmailTextBox" ValidationExpression=".*@.*\..*" ErrorMessage="Email address is invalid" EnableClientScript="true" runat="server" ValidationGroup="Insert">*</asp:RegularExpressionValidator>

Or, if you want to do it in code rather than use a control, try this:

public static bool isEmail(string inputEmail)
{
    if( inputEmail == null || inputEmail.Length == 0 )
    {
        throw new ArgumentNullException( "inputEmail" );
    }

    const string expression = @"^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}" +
                              @"\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\" +
                              @".)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$";

    Regex regex = new Regex(expression);
    return regex.IsMatch(inputEmail);
}