"""Validation utilities."""
import re

def validate_email(email: str) -> bool:
    """Validate email format."""
    if not email:
        return False
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return bool(re.match(pattern, email))

def validate_username(username: str) -> bool:
    """Validate username format."""
    if not username:
        return False
    # Username should be 3-20 characters, alphanumeric and underscores only
    pattern = r'^[a-zA-Z0-9_]{3,20}$'
    return bool(re.match(pattern, username))

def validate_password(password: str) -> bool:
    """Validate password strength."""
    if not password:
        return False
    # At least 8 characters, contains uppercase, lowercase, and number
    if len(password) < 8:
        return False
    has_upper = bool(re.search(r'[A-Z]', password))
    has_lower = bool(re.search(r'[a-z]', password))
    has_digit = bool(re.search(r'\d', password))
    return has_upper and has_lower and has_digit