true, 'alincoln69' => true, 'hd3' => true, 'Alincoln' => true, 'a.lincoln' => true, 'alincoln!' => false, '' => false, // These are silly, but permitted. '7' => true, '0' => true, '____' => true, '-' => true, // These are not permitted because they make capturing @mentions // ambiguous. 'joe.' => false, // We can never allow these because they invalidate usernames as tokens // in commit messages ("Reviewers: alincoln, usgrant"), or as parameters // in URIs ("/p/alincoln/", "?user=alincoln"), or make them unsafe in // HTML. Theoretically we escape all the HTML/URI stuff, but these // restrictions make attacks more difficult and are generally reasonable, // since usernames like "<^, ,^>" don't seem very important to support. '