MediaWiki should permit all lowercase usernames. There is [[mw:$wgCapitalLinkOverrides]], but it currently doesn't apply to User:, Special: and MediaWiki: namespaces.

Per IRC (MediaWiki-General):

<route> i've just modded 1.20.5 to permit all lowercase usernames, to do this i've removed NS_USER from the $alwaysCapitalizedNamespaces array in Namespace.php and i've removed calls to $wgContLang->ucfirst( $name ) in User.php. i've created a user and it seems to have worked as i'd expect -- is there anything i might be overlooking that could cause problems in future?

This was first allowed when the feature was implemented (r57558), then disallowed back (r57559, r58401, r60063) with unclear comments about MediaWiki crashing. This was back in 2009, though, and a lot might have changed.

The user provided the patch he used: http://pastebin.com/1C1iTbpU

Version: 1.22.0
Severity: enhancement