Check for likely incorrectly migrated django.urls.path() calls (that still have regex characters in the route)

[Chris Lamb]

I recently moved a project to Django 2.x, including moving to django.urls.path. However, I wasn't careful enough in the details so ended up migrating, for example:

    from django.conf.urls import url

    url('^faq$', views.faq,
        name='faq'),

to

    from django.urls import path

    path('^faq$', views.faq,
         name='faq'),

This doesn't cause any errors (!) but results in, for example, reverse('faq') returning the (encoded) url /%5Efaq$.

As this is a) likely to be very common during the migration with a high cost of a true positive, combined with b) the fact that the chance of a false positive is fairly low, I suggest we add a check for this.

(PR incoming...)

[Chris Lamb]

PR with tests: ​https://github.com/django/django/pull/9183

[Chris Lamb]

PR updated after review.

[Tim Graham <timograham@…>]

In 998c9dd:

Fixed #28663 -- Add a check for likely incorrectly migrated django.urls.path() routes.

[Tim Graham <timograham@…>]

In 518c1135:

[2.0.x] Fixed #28663 -- Add a check for likely incorrectly migrated django.urls.path() routes.

Backport of 998c9dd599cd907bb38f440fff13a808571589f8 from master