Email Validation: converting a regular expression written in PHP (preg) to .NET (Regex)

Answer 1

.NET regular expression syntax is not the same as in PHP, and Regex is the only built-in class to use regular expression (but there might be other third party implementation). Anyway, it's pretty easy to validate an email address with Regex... straight from the source

^([0-9a-zA-Z]([-\.\w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,9})$

Answer 2

The problem with your regular expression in .NET is that the possessive quantifiers aren't supported. If you remove those, it works. Here's the regular expression as a C# string:

@"^[-_a-z0-9\'+*$^&%=~!?{}]+(?:\.[-_a-z0-9\'+*$^&%=~!?{}]+)*@(?:(?![-.])[-a-z0-9.]+(?<![-.])\.[a-z]{2,6}|\d{1,3}(?:\.\d{1,3}){3})(?::\d+)?$"

Here's a test bed for it based on the page you linked to, including all the strings that should match and the first three of those that shouldn't:

using System;
using System.Text.RegularExpressions;

public class Program
{
    static void Main(string[] args)
    {
        foreach (string email in new string[]{
            "[email protected]",
            "[email protected]",
            "hasApostrophe.o'[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]:25",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "[email protected]",
            "&*=?^+{}'[email protected]",
            "[email protected]",
            "@missingLocal.org",
            "missingatSign.net"
        })
        {
            string s = @"^[-_a-z0-9\'+*$^&%=~!?{}]+(?:\.[-_a-z0-9\'+*$^&%=~!?{}]+)*@(?:(?![-.])[-a-z0-9.]+(?<![-.])\.[a-z]{2,6}|\d{1,3}(?:\.\d{1,3}){3})(?::\d+)?$";
            bool isMatch = Regex.IsMatch(email, s, RegexOptions.IgnoreCase);
            Console.WriteLine(isMatch);
        }
    }
}

Output:

True
True
True
True
True
True
True
True
True
True
True
True
True
True
True
True
True
True
False
False
False

A problem though is that it fails to match some valid email-addresses, such as foo\@[email protected]. It's better too match too much than too little.

Answer 3

I've used this function before in a bunch of e-commerce applications and never had a problem.

    public static bool IsEmailValid(string emailAddress)
    {
        Regex emailRegEx = new Regex(@"\b[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b");
        if (emailRegEx.IsMatch(emailAddress))
        {
            return true;
        }

        return false;
    }

Answer 4

You really shouldn't be using a RegEx to parse email addresses in .NET. Your better option is to use the functionality built into the framework.

Try to use your email string in the constructor of the MailAddress class. If it throws a FormatException then the address is no good.

try 
{
    MailAddress addr = new MailAddress("[email protected]")
    // <- Valid email if this line is reached
}
catch (FormatException)
{
    // <- Invalid email if this line is reached
}

You can see an answer a Microsoft developer gave to another email validation question, where he explains how .NET's email parsing has also improved dramatically in .NET 4.0. Since at the time of answering this, .NET 4.0 is still in beta, you probably aren't running it, however even previous versions of the framework have adequate email address parsing code. Remember, in the end you're most likely going to be using the MailAddress class to send your email anyway. Why not use it to validation your email addresses. In the end, being valid to the MailAddress class is all that matters anyway.