Alphabetic characters and dot (example: 'D.'):

public function rules(): array
{
    return [
        'initials' => [
            'regex:/^[A-Za-z.]+$/'
        ]
    ]
}

Alphabetic characters and space (example: 'van der'):

public function rules(): array
{
    return [
        'insertion' => [
            'regex:/^[A-Za-z ]+$/'
        ]
    ]
}