what does an '_' in django url means like,
url(_(r'^mylink/'), include('link5.urls')),
_ plus a string should be an error but one public app is using such construct
_ is often a shortcut for a gettext function that will produce a localized version of the string. It's unusual to do it for URLs, but it's documented: https://docs.djangoproject.com/en/1.11/topics/i18n/translation/#translating-url-patterns
At the top of the file you might find something like:
from django.utils.translation import ugettext as _