JAVASCRIPT

regex for username

/^[a-zA-Z0-9_-]{3,16}$/

username regex

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

regex for username

# works in most newer browsers
^(?=.{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

# works in all browsers, but does the same as the above RegEx
^(?=[a-zA-Z0-9._]{8,20}$)(?!.*[_.]{2})[^_.].*[^_.]$

regex for username

let username = '';
username = username.replace(/s/g,'_');
username = username.replace(/-/g,'.');
username = username.match(/[a-zA-Z0-9.s]+/g).join('_');

simple username regex

^[A-Za-z][A-Za-z0-9_]{2,16}$

regex for good username

let userCheck = /^[a-z][a-z]+d*$|^[a-z]dd+$/i;

Regex checking username

#!/usr/bin/env python3

import re

def validate_user(username, minlen):
    """Checks if the received username matches the required conditions."""
    if type(username) != str:
        raise TypeError("username must be a string")
    if minlen < 1:
        raise ValueError("minlen must be at least 1")
    
    # Usernames can't be shorter than minlen
    if len(username) < minlen:
        return False
    # Usernames can only use letters, numbers, dots and underscores
    if not re.match('^[a-z0-9._]*$', username):
        return False
    # Usernames can't begin with a number
    if username[0].isnumeric():
        return False
    return True