Add custom NamespaceNameValidator

app/models/namespace.rb

@@ -23,10 +23,10 @@ class Namespace < ActiveRecord::Base
   validates :owner, presence: true, unless: ->(n) { n.type == "Group" }
   validates :name,
-    presence: true, uniqueness: true,
     length: { within: 0..255 },
-    format: { with: Gitlab::Regex.namespace_name_regex,
-              message: Gitlab::Regex.namespace_name_regex_message }
+    namespace_name: true,
+    presence: true,
+    uniqueness: true
   validates :description, length: { within: 0..255 }
   validates :path,

app/validators/namespace_name_validator.rb

+# NamespaceNameValidator
+#
+# Custom validator for GitLab namespace name strings.
+class NamespaceNameValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    unless value =~ Gitlab::Regex.namespace_name_regex
+      record.errors.add(attribute, Gitlab::Regex.namespace_name_regex_message)
+    end
+  end
+end