Input validation is the process of verifying that user-supplied data meets expected criteria before processing it. It's crucial because unvalidated input is the root cause of many security vulnerabilities including injection attacks, buffer overflows, and data corruption.

Key principles:

• Whitelist validation: Define what is acceptable rather than what isn't
• Server-side validation: Never rely solely on client-side validation
• Sanitization: Clean or encode input when validation isn't sufficient
• Length limits: Prevent buffer overflows and DoS attacks

Example of proper validation:

import re

def validate_email(email):
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    if re.match(pattern, email) and len(email) <= 254:
        return True
    return False