<p>The error "Display Name is invalid" while registering might not be too helpful. Instead, display which characters are not allowed (I'm not completely sure about the handling of <code>\/</code>):</p>

<div class="highlight"><pre><span class="gh">diff --git a/app/models/user.rb b/app/models/user.rb</span>
<span class="gh">index 3b43130..27662f1 100644</span>
<span class="gd">--- a/app/models/user.rb</span>
<span class="gi">+++ b/app/models/user.rb</span>
<span class="gu">@@ -36,7 +36,7 @@ class User < ActiveRecord::Base</span>
   validates_length_of :display_name, :within => 3..255, :allow_nil => true
   validates_email_format_of :email, :if => Proc.new { |u| u.email_changed? }
   validates_email_format_of :new_email, :allow_blank => true, :if => Proc.new { |u| u.new_email_changed? }
<span class="gd">-  validates_format_of :display_name, :with => /^[^\/;.,?%#]*$/, :if => Proc.new { |u| u.display_name_changed? }</span>
<span class="gi">+  validates_format_of :display_name, :with => /^[^\/;.,?%#]*$/, :message => "contains invalid characters (\/;.,?%#)" :if => Proc.new { |u| u.display_name_changed? }</span>
   validates_format_of :display_name, :with => /^\S/, :message => "has leading whitespace", :if => Proc.new { |u| u.display_name_changed? }
   validates_format_of :display_name, :with => /\S$/, :message => "has trailing whitespace", :if => Proc.new { |u| u.display_name_changed? }
   validates_numericality_of :home_lat, :allow_nil => true
</pre></div>

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