c# - How to allow only double backslash in email with Regex?

I am trying to accept \\ and / before the @ symbol in our email like usernames.

Below is the current Regex that we are using:

^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$

The reason why I only want to allow double backslash is that I want the string to be escaped by the time I use this regex to validate if it is a valid username.

Valid and Invalid Use Cases:

1. hello\\world@email.com is valid.



2. hello\world@email.com is invalid.


3. hello/world@gmil.com is valid.


4. hello@email.com is valid.

I have tried the following regular expression but it allowed single backslash also:

^([\w-(\\\\)/\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$

Answer

How it should be escaped depends a little on what language you are using, but here you go:

^(([-\w\/\.]|\\\\)+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$

See it in action

All I did is add a forward slash in the first group \/ and add two back slashes as an alternative |\\\\.

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Here is the C# escaped version:

^(([-\\w\\/\\.]|\\\\\\\\)+)@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.)|(([\\w-]+\\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\\]?)$