没有完美的正则表达式，但我认为，基于对RFC5322的研究，这个正则表达式相当强大。用c#字符串插值，我认为也很容易理解。

const string atext = @"a-zA-Z\d!#\$%&'\*\+-/=\?\^_`\{\|\}~";
var localPart = $"[{atext}]+(\\.[{atext}]+)*";
var domain = $"[{atext}]+(\\.[{atext}]+)*";
Assert.That(() => EmailRegex = new Regex($"^{localPart}@{domain}$", Compiled), 
Throws.Nothing);

用NUnit 2.x检查。

1

^[\w!#$%&'*+\-/=?\^_`{|}~]+(\.[\w!#$%&'*+\-/=?\^_`{|}~]+)*@((([\-\w]+\.)+[a-zA-Z]{2,4})|(([0-9]{1,3}\.){3}[0-9]{1,3}))$

2

^(([^<>()[\]\\.,;:\s@\""]+(\.[^<>()[\]\\.,;:\s@\""]+)*)|(\"".+\""))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$

没有完美的正则表达式，但我认为，基于对RFC5322的研究，这个正则表达式相当强大。用c#字符串插值，我认为也很容易理解。

const string atext = @"a-zA-Z\d!#\$%&'\*\+-/=\?\^_`\{\|\}~";
var localPart = $"[{atext}]+(\\.[{atext}]+)*";
var domain = $"[{atext}]+(\\.[{atext}]+)*";
Assert.That(() => EmailRegex = new Regex($"^{localPart}@{domain}$", Compiled), 
Throws.Nothing);

用NUnit 2.x检查。

这并不满足rfc 5321和5322的所有要求，但它适用于以下定义。

@"^([0-9a-zA-Z]([\+\-_\.][0-9a-zA-Z]+)*)+"@(([0-9a-zA-Z][-\w]*[0-9a-zA-Z]*\.)+[a-zA-Z0-9]{2,17})$";

下面是代码

const String pattern =
   @"^([0-9a-zA-Z]" + //Start with a digit or alphabetical
   @"([\+\-_\.][0-9a-zA-Z]+)*" + // No continuous or ending +-_. chars in email
   @")+" +
   @"@(([0-9a-zA-Z][-\w]*[0-9a-zA-Z]*\.)+[a-zA-Z0-9]{2,17})$";

var validEmails = new[] {
        "ma@hostname.com",
        "ma@hostname.comcom",
        "MA@hostname.coMCom",
        "m.a@hostname.co",
        "m_a1a@hostname.com",
        "ma-a@hostname.com",
        "ma-a@hostname.com.edu",
        "ma-a.aa@hostname.com.edu",
        "ma.h.saraf.onemore@hostname.com.edu",
        "ma12@hostname.com",
        "12@hostname.com",
};
var invalidEmails = new[] {
        "Abc.example.com",     // No `@`
        "A@b@c@example.com",   // multiple `@`
        "ma...ma@jjf.co",      // continuous multiple dots in name
        "ma@jjf.c",            // only 1 char in extension
        "ma@jjf..com",         // continuous multiple dots in domain
        "ma@@jjf.com",         // continuous multiple `@`
        "@majjf.com",          // nothing before `@`
        "ma.@jjf.com",         // nothing after `.`
        "ma_@jjf.com",         // nothing after `_`
        "ma_@jjf",             // no domain extension 
        "ma_@jjf.",            // nothing after `_` and .
        "ma@jjf.",             // nothing after `.`
    };

foreach (var str in validEmails)
{
    Console.WriteLine("{0} - {1} ", str, Regex.IsMatch(str, pattern));
}
foreach (var str in invalidEmails)
{
    Console.WriteLine("{0} - {1} ", str, Regex.IsMatch(str, pattern));
}

作为Alex的流行答案的更新:在。net 5中，MailAddress现在有一个TryCreate。所以你可以这样做:

public static bool IsValidEmail(string email)
{
    if (!MailAddress.TryCreate(email, out var mailAddress))
        return false;

    // And if you want to be more strict:
    var hostParts = mailAddress.Host.Split('.');
    if (hostParts.Length == 1)
        return false; // No dot.
    if (hostParts.Any(p => p == string.Empty))
        return false; // Double dot.
    if (hostParts[^1].Length < 2)
        return false; // TLD only one letter.

    if (mailAddress.User.Contains(' '))
        return false;
    if (mailAddress.User.Split('.').Any(p => p == string.Empty))
        return false; // Double dot or dot at end of user part.

    return true;
}

这个正则表达式工作得很完美:

bool IsValidEmail(string email)
{
    return Regex.IsMatch(email, @"^[\w!#$%&'*+\-/=?\^_`{|}~]+(\.[\w!#$%&'*+\-/=?\^_`{|}~]+)*@((([\-\w]+\.)+[a-zA-Z]{2,4})|(([0-9]{1,3}\.){3}[0-9]{1,3}))\z");
}