Why it is not possible to add . to a partial URL in CRM portal

There is no reference in microsoft documentation that we cannot use dot a string in partial url in crm portal. But still it does not allow me to include . in the partial URL. Can anyone explain what is the reason behind this. Since even when i try to save a partial URL with dot in it, it errors out saying "Partial URL of a child page cannot be'/'. Please enter a partial URL with letters,number and hyphen or underscores."