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