正则表达式可以验证用分号分隔的电子邮件列表？

Question

I have the following c# extension method for validating an email address. The regex came from Microsoft on their How to: Verify that Strings Are in Valid Email Format page.

I need to improve this method to be able to handle a semi-colon seperated list of emails. An valid example string could be as badly formatted as: ";; ; ; xxx.sss.xxx ; ;; xxx.sss.xxx;"

    /// <summary>
    /// Validates the string is an Email Address...
    /// </summary>
    /// <param name="emailAddress"></param>
    /// <returns>bool</returns>
    public static bool IsValidEmailAddress(this string emailAddress)
    {
        var valid = true;
        var isnotblank = false;

        var email = emailAddress.Trim();
        if (email.Length > 0)
        {
            isnotblank = true;
            valid = Regex.IsMatch(email, @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
            @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$", RegexOptions.IgnoreCase);
        }

        return (valid && isnotblank);
    }

Answer 1

Microsoft's regex does a pretty good job. However, it doesn't catch a few strange scenarios and a number of special characters which are valid for email. I'll give you a different regex. Choose to use it or not is your prerogative.

I would separate the concerns by having one extension method which validates an email address and another which validates the list. Do a .trim() on each email before passing it to the email validation method. So, something like this:

    /// <summary>
    /// Validates the string is an Email Address...
    /// </summary>
    /// <param name="emailAddress"></param>
    /// <returns>bool</returns>
    public static bool IsValidEmailAddress(this string emailAddress)
    {
        var valid = true;
        var isnotblank = false;

        var email = emailAddress.Trim();
        if (email.Length > 0)
        {
            // Email Address Cannot start with period.
            // Name portion must be at least one character
            // In the Name, valid characters are:  a-z 0-9 ! # _ % & ' " = ` { } ~ - + * ? ^ | / $
            // Cannot have period immediately before @ sign.
            // Cannot have two @ symbols
            // In the domain, valid characters are: a-z 0-9 - .
            // Domain cannot start with a period or dash
            // Domain name must be 2 characters.. not more than 256 characters
            // Domain cannot end with a period or dash.
            // Domain must contain a period
            isnotblank = true;
            valid = Regex.IsMatch(email, @"\A([\w!#%&'""=`{}~\.\-\+\*\?\^\|\/\$])+@{1}\w+([-.]\w+)*\.\w+([-.]\w+)*\z", RegexOptions.IgnoreCase) &&
                !email.StartsWith("-") &&
                !email.StartsWith(".") &&
                !email.EndsWith(".") && 
                !email.Contains("..") &&
                !email.Contains(".@") &&
                !email.Contains("@.");
        }

        return (valid && isnotblank);
    }

    /// <summary>
    /// Validates the string is an Email Address or a delimited string of email addresses...
    /// </summary>
    /// <param name="emailAddress"></param>
    /// <returns>bool</returns>
    public static bool IsValidEmailAddressDelimitedList(this string emailAddress, char delimiter = ';')
    {
        var valid = true;
        var isnotblank = false;

        string[] emails = emailAddress.Split(delimiter);

        foreach (string e in emails)
        {
            var email = e.Trim();
            if (email.Length > 0 && valid) // if valid == false, no reason to continue checking
            {
                isnotblank = true;
                if (!email.IsValidEmailAddress())
                {
                    valid = false;
                }
            }
        }
        return (valid && isnotblank);
    }

Answer 2

You can use the following regex to allow several emails separated by semicolon and (optionally) spaces besides the semicolon. It will also accept single email address that does not end in semicolon.

Right now it allows empty entries as well(no email addresses). If You wanna change it to have at least 1, then replace the final * by + to require at least one address.

(([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)(\s*;\s*|\s*$))*

If you want to allow comma as well as semicolon, you can chage following :

(\s*;\s*|\s*$)

to this:

(\s*(;|,)\s*|\s*$)