Email regex used Jquery does not work in .Net C#

Question

CONTEXT: I have an email regex that is used in jquery while registration of users which goes as follows. It was already present in the database before I began with the project and I assume it is correct:

/^(?!.*\.{2})[A-Z0-9_%][\w\.\+\-]*[a-zA-Z0-9._%]@[a-zA-Z0-9]([\w\.\-]+)((\.([a-zA-Z]){2,4})+)$/im

After validating on the frontend, the registration process calls an API which validates the email using Fluent Validation.

REQUIREMENT:

Through jquery, a certain email (testuser.@domain.com) is successfully validated but the fluent validation disapproves it. There is a requirement to get such email successfully validated and registered.

ISSUE: I found that we can pass our custom regex in the Fluent Validation itself using the below code:

var RegisterEmailRegexPatternstring = userSettingService.CurrentUserSettings.RegisterEmailRegexPattern.Replace("/^","").Replace("/im","") ?? @"^(?!.*\.{2})[A-Z0-9_%][\w\.\+\-]*[a-zA-Z0-9_%]@[a-zA-Z0-9]([\w\-]+)((\.([a-zA-Z]){2,4})+)$";
            
var RegisterEmailRegexPattern = new Regex(RegisterEmailRegexPatternstring);
            
RuleFor(x => x.email).Matches(RegisterEmailRegexPattern);

But the regex which is used to validate in jquery fails in .NET/C# and I'm unable to find the reason behind it.

QUESTION:

1. Why does a regex which works in jquery but fails in .NET/C#?
2. What can be done to prevent this issue in this case?

Answer 1

There are some differences between the regex engines used by jQuery and .NET/C#, which may cause some patterns to work differently or not at all. Some of the common differences are:

jQuery uses the JavaScript regex engine, which does not support named capture groups, lookbehind assertions, or Unicode character classes. .NET/C# supports all of these features and more.

jQuery uses the /g flag to indicate a global search, which means it will find all matches in the input string. .NET/C# does not use flags, but has methods such as Regex.Matches or Regex.Replace that can perform global searches.

jQuery uses the /i flag to indicate a case-insensitive search, which means it will ignore the difference between uppercase and lowercase letters. .NET/C# uses the RegexOptions.IgnoreCase enumeration value to achieve the same effect.

jQuery uses the /m flag to indicate a multiline search, which means it will treat the ^ and $ anchors as matching the start and end of each line, rather than the whole input string. .NET/C# uses the RegexOptions.Multiline enumeration value to achieve the same effect.

In your case, I think the problem is caused by the /m flag in your jQuery regex. This flag makes the ^ and $ anchors match the start and end of each line in the input string, rather than the whole string. This means that your regex will accept an email that has a newline character (\n) in it, such as testuser.@domain.com\n. However, this is not a valid email address, and it will fail in .NET/C#, which does not use the /m flag by default.

To fix this issue, you can either remove the /m flag from your jQuery regex or add the RegexOptions.Multiline value to your .NET/C# regex. This will make both regex engines behave consistently and reject invalid emails that contain newline characters.

You can use this approach to validate email in C#:

public static Regex EmailRegex =>
            new(@"\A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)\Z", RegexOptions.IgnoreCase);

bool isValidEmail = EmailRegex.IsMatch("test@gmail.com");