域名备案时域名格式不正确

import re

def validate_domain(domain):
    # 域名后缀检查
    valid_tlds = ['.com', '.cn', '.net']
    if not any(domain.endswith(tld) for tld in valid_tlds):
        return False, "Invalid domain suffix"
    
    # 域名字符检查
    if not re.match(r'^[a-z0-9-]+(\.[a-z0-9-]+)*\.[a-z]{2,}$', domain, re.IGNORECASE):
        return False, "Invalid domain characters"
    
    # 域名长度检查
    if len(domain) > 253:
        return False, "Domain length exceeds limit"
    
    return True, "Domain is valid"

# 测试
domain = "example.com"
is_valid, message = validate_domain(domain)
print(message)