Regular expression to validate username

Question

I'm trying to create a regular expression to validate usernames against these criteria:

1. Only contains alphanumeric characters, underscore and dot.
2. Underscore and dot can't be at the end or start of a username (e.g _username / username_ / .username / username.).
3. Underscore and dot can't be next to each other (e.g user_.name).
4. Underscore or dot can't be used multiple times in a row (e.g user__name / user..name).
5. Number of characters must be between 8 to 20.

This is what I've done so far; it sounds it enforces all criteria rules but the 5th rule. I don't know how to add the 5th rule to this:

 ^[a-zA-Z0-9]+([._]?[a-zA-Z0-9]+)*$ 

Answer 1

^(?=.{8,20}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$  └─────┬────┘└───┬──┘└─────┬─────┘└─────┬─────┘ └───┬───┘        │         │         │            │           no _ or . at the end        │         │         │            │        │         │         │            allowed characters        │         │         │        │         │         no __ or _. or ._ or .. inside        │         │        │         no _ or . at the beginning        │        username is 8-20 characters long 

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If your browser raises an error due to lack of negative look-behind support, use the following alternative pattern:

^(?=[a-zA-Z0-9._]{8,20}$)(?!.*[_.]{2})[^_.].*[^_.]$ 

Answer 2

As much as I love regular expressions I think there is a limit to what is readable

So I would suggest

new Regex("^[a-z._]+$", RegexOptions.IgnoreCase).IsMatch(username) && !username.StartsWith(".") && !username.StartsWith("_") && !username.EndsWith(".") && !username.EndsWith("_") && !username.Contains("..") && !username.Contains("__") && !username.Contains("._") && !username.Contains("_."); 

It's longer but it won't need the maintainer to open expresso to understand.

Sure you can comment a long regex but then who ever reads it has to rely on trust.......