Serverfault (and other Stack Exchange sites) provide a search function. One of the search modifiers for the search function is the 'user:' modifier. It looks like though, that this modifier fails to work with user names (the modifier only works with 'numbers uniquely assigned to each user'). Is forbidding to search the site by usernames on purpose or there actually exist some functionality that I don't know of?
I.e.
user:trismarck
fails
user:104139
works