use standard gitlab namespace regex for group name validation

app/views/shared/_group_form.html.haml

@@ -13,8 +13,9 @@
       .input-group-addon
         = root_url
       = f.text_field :path, placeholder: 'open-source', class: 'form-control',
-        autofocus: local_assigns[:autofocus] || false,  pattern: "[a-zA-Z0-9-_\\.]+",
-        required: true, title: 'Please choose a group name with no special characters.'
+        autofocus: local_assigns[:autofocus] || false, required: true,
+        pattern: Gitlab::Regex::NAMESPACE_REGEX_STR_SIMPLE,
+        title: 'Please choose a group name with no special characters.'
     - if @group.persisted?
       .alert.alert-warning.prepend-top-10