Regular expression to validate username

Issue

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]+)*$

Solution

^(?=.{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}$)(?!.*[_.]{2})[^_.].*[^_.]$

Answered By - Ωmega