Basic username validation. Checks for profanity, emails, and other basic rules.

validate_username.py

import re 

from better_profanity import profanity
from pydantic import BaseModel
from email_validator import validate_email, EmailNotValidError


MINIMUM_USERNAME_LENGTH = 3
MAXIMUM_USERNAME_LENGTH = 24
ALLOWED_CHARACTERS_REGEX = re.compile(r"^[\x20-\x7E]+$")  # Only allow visible/printable ASCII characters
RESERVED_WORDS = ["admin", "administrator", "staff", "support", "root"]


class UsernameValidationResult(BaseModel):
    is_valid: bool
    reason: str | None = None


def validate_username(username: str) -> UsernameValidationResult:
    if len(username) < MINIMUM_USERNAME_LENGTH:
        return UsernameValidationResult(is_valid=False, reason=f"Username is too short. It must be at least {MINIMUM_USERNAME_LENGTH} characters.")

    if len(username) > MAXIMUM_USERNAME_LENGTH:
        return UsernameValidationResult(is_valid=False, reason=f"Username is too long. It must be at most {MAXIMUM_USERNAME_LENGTH} characters.")

    if username.strip() != username:
        return UsernameValidationResult(is_valid=False, reason=f"Username cannot start or end with whitespace.")

    if not ALLOWED_CHARACTERS_REGEX.match(username):
        return UsernameValidationResult(
            is_valid=False, reason=f"Username contains invalid characters. Only alphanumeric characters, underscores, and hyphens are allowed."
        )

    if username.isdigit():
        return UsernameValidationResult(is_valid=False, reason=f"Username cannot be number-only.")

    if username.lower() in RESERVED_WORDS:
        return UsernameValidationResult(is_valid=False, reason=f"Username is reserved.")

    if profanity.contains_profanity(username):
        return UsernameValidationResult(is_valid=False, reason=f"Username might contain profanity.")

    try:
        validate_email(username, check_deliverability=False)  # check_deliverability=False makes it not require internet
        return UsernameValidationResult(is_valid=False, reason=f"Username cannot be an email address.")
    except EmailNotValidError as e:
        pass  # Username is not an email address

    return UsernameValidationResult(is_valid=True)