Template:IsUserRegistered/doc

Purpose
Detects if this template is placed anywhere within the user namespace of a registered user, IPv4 user or IPv6 user. Unlike Template:IP-talk, this works even if the user's username starts with numbers and can differentiate between IPv4 and IPv6.

Usage
This template should not be used outside the user namespace and will behave as if placed in a registered user's namespace if used elsewhere.

Limitations
This template will fail if the user's username looks just like an IP address, but such usernames are a violation of the username policy.