Strict Email Validation Regex Pattern

// JavaScript implementation
const emailRegex = /^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/;

function isValidEmail(email) {
  if (!email || typeof email !== 'string') return false;
  if (email.length > 254) return false; // RFC limit
  return emailRegex.test(email);
}

// Usage examples
console.log(isValidEmail('user@example.com')); // true
console.log(isValidEmail('invalid.email')); // false

# Python implementation
import re

EMAIL_REGEX = re.compile(
    r'^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$'
)

def is_valid_email(email):
    if not email or not isinstance(email, str):
        return False
    if len(email) > 254:  # RFC limit
        return False
    return bool(EMAIL_REGEX.match(email))

# Usage examples
print(is_valid_email('user@example.com'))  # True
print(is_valid_email('invalid.email'))     # False

<?php
// PHP implementation
function isValidEmail($email) {
    if (!is_string($email) || strlen($email) > 254) {
        return false;
    }
    
    $pattern = '/^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/';
    return preg_match($pattern, $email) === 1;
}

// Usage examples
var_dump(isValidEmail('user@example.com')); // bool(true)
var_dump(isValidEmail('invalid.email'));    // bool(false)
?>