EMail Address Validation in C# and ASP.NET Core A lightweight and customizable helper class to validate any e-mail address using the HTML living standards RegEx and/or ASP.NET Core built-in validators in C#

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Validating an email address is a classic requirement of almost any web application. Most of the modern client-side and server-side web frameworks provide native methods to fulfill this need: however, as many web developers know all too well, the methodologies used for validating email addresses do not always return the same result - and what is considered "valid" for them does not always correspond to the desired result for our specific case.

To better understand such concept, consider the following email addresses:

The above samples are taken from RFC 3696, Application Techniques for Checking and Transformation of Names, written by the author of the SMTP protocol (RFC 2821) as a human readable guide to SMTP.

As strange as it might seem, all the above e-mail addresses are "valid": or at least they were, according to RFC 2822 section 3.4.1, until it was obsoleted by  RFC 5322. However, even RFC 5322  takes as "valid" email addresses using a syntax that are widely considered to be simultaneously too strict (before the "@" character), too vague (after the "@" character), and too lax (allowing comments, whitespace characters, and quoted strings).

Here's another small list of "valid" email address as per RFC 5322:

  • joe.blow@[IPv6:2001:db8::1]
  • joe.blow(comment)
  • joe.blow(comment)@(another comment with spaces)
  • "Joe..Blow"

... and so on. As we can see, we're still far from what we need for practical use.

An "almost perfect" solution came with the release of the HTML living standards, which introduced a definition based upon a "willful violation" of RFC 5322 to overcome the above issues. The new definition was even backed up with a JavaScript and Perl-compatible regular expression that can be used to properly implement it:

The above Regex is good enough to cut out most of the "odd address" above - and that's the reason why I've used it for a lot of my personal and business apps, as well as suggesting it to my fellow developers or colleagues. The only real issue I have with it  with th  it still allows the following:

  • admin@localhost
  • abc@cba

As a matter of fact, there is nothing wrong with the above e-mail addresses: such "dot-less" format is definitely valid and do have sense in most scenarios - for example, if we need to support "intranet" e-mail addresses or similar scenarios. However, when implementing a web-based service for external users, we might want to exclude those "dot-less" e-mail address from the valid ones.

For that very reason, I've ended up implementing my own C# helper class that can be used to validate e-mail addresses with or without dots.

Here's the source code:

IMPORTANT: the above code has been copied & pasted here for reference purposes only. To download the latest and most recently updated version, check out the project's repository on GitHub.

As you can see, the validation process relies upon the IsValidEmailAddress static function, which accepts the following parameters:

  • email : the e-mail address to check / validate
  • useRegEx : TRUE to use the HTML5 living standard e-mail validation RegEx, FALSE to use the built-in validator provided by .NET (default: FALSE)
  • requireDotInDomainName : TRUE to only validate e-mail addresses containing a dot in the domain name segment, FALSE to allow "dot-less" domains (default: FALSE)

The above code has been released under MIT license, meaning that you're free to use it for any project or use it to develop your own e-mail validatior function.


That's basically it: if you like the above code, feel free to give us a feedback in the comments section of this post.

Fork me on GitHub


About Ryan

IT Project Manager, Web Interface Architect and Lead Developer for many high-traffic web sites & services hosted in Italy and Europe. Since 2010 it's also a lead designer for many App and games for Android, iOS and Windows Phone mobile devices for a number of italian companies. Microsoft MVP for Development Technologies since 2018.

View all posts by Ryan

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