// change these value to test your condition;
$bb_name = 'somebank';
$password = '123456';
$confirm = '123456';
$email = 'john@abc.com';
$city = 'some city';
$reg_number = '1234';

$check = true;

if(empty($bb_name) || empty($city) || empty($reg_number) || empty($email) || empty($password) || empty($confirm)) {
    $check = false;
    $errors[] = 'missing field';
}

if (!preg_match('/^[a-zA-Z]+$/',$bb_name)) {
    $check = false;
    $errors[] = 'preg_match failed';
}

if (strlen($password) < 5 || $password !== $confirm) {
    $check = false;
    $errors[] = 'password length failed or mismatch';
}

if (filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
    $check = false;
    $errors[] = 'invalid email';
}

if (!$check) {
    foreach ($errors as $error) {
        echo $error.'<br>';
    }
} else {
    $query="INSERT INTO bank_reg (bank_name,city,number_reg,email,password) VALUES ('$bb_name','$city','$reg_number','$email','".md5($password)."')";
    echo $query;
}