Summary: Cloud and Datacenter Management MVP, Thomas Rayner, shows how use regex to detect if a string is a valid Active Directory user name.

The invalid characters for an Active Directory user name are /\[:;|=,+*?<>]\ and “. How do I tell if a string is a valid Active Directory user name?

You can treat this array of characters as a pattern and see if they exist within the string

$questionablestring -match "[$([Regex]::Escape('/\[:;|=,+*?<>') + '\]' + '\"')]"

You’ll have to manually escape ] and “ because [regex]::escape() is a bit quirky.