How Django resolve this non-greedy regex url pattern?

Question

I'm following a tutorial which works with Django, there is a URL pattern like this:

url(r'^lists/(.+)/$', views.list_view, name='list_view')

That's a greedy regex pattern and it will match something such as: lists/123/add_item/ with 123/add_item as matched-group. That is not what is expected. So I tried to change it to non-greedy one like this:

url(r'^lists/(.+?)/$', views.list_view, name='list_view')

But Django still resolve it to views.list_view

Could you please give me some explanation?

Possible duplicate of [What do ^ and $ mean in a regular expression?](https://stackoverflow.com/questions/6908725/what-do-and-mean-in-a-regular-expression) — Wiktor Stribiżew, May 11 '19 at 22:15

Exelian · Answer 1 · 2017-08-10T08:11:08.963

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You should remove the trailing $ from your regex, since that will always match the final / instead of the next.

I think the fact that Django does match the 2nd URL is due to Django not matching the URL but instead using a re.search. You can see this behaviour here: Django source

edited Aug 10 '17 at 08:11

answered Aug 10 '17 at 07:39

Exelian

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Your answer seems explained regex part. but Django still resolve `lists/123/add_item` to `views.list_view` – dvn0zzz Aug 10 '17 at 08:05
I also guess that @daniel's answer is strictly the more correct one. Although I'd highly recommend browsing the django source to understand more of what happens on the inside. – Exelian Aug 10 '17 at 08:12

wencakisa · Answer 2 · 2017-08-10T07:35:21.917

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Explanation of ^lists/(.+?)/$:

^ asserts position at start of the string
lists matches the characters lists literally (case sensitive)
/ matches the character / literally (case sensitive)
1st Capturing Group (.+?)
- .+? matches any character (except for line terminators)
- +? Quantifier — Matches between one and unlimited times, as few times as possible, expanding as needed (lazy)
/ matches the character / literally (case sensitive)
$ means end of string

Credits: http://regex101.com

But I didn't understand what you want exactly to match your pattern

UPDATE:

Helpful link: https://docs.python.org/3/howto/regex.html#greedy-versus-non-greedy

edited Aug 10 '17 at 07:35

answered Aug 10 '17 at 07:32

wencakisa

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Hi! with that pattern, I was expecting it will match only: lists/123/ but not lists/123/add_item/ – dvn0zzz Aug 10 '17 at 07:34
If you want to match a number only (probably an `id`), you can use `^lists/(?P[0-9]+)/$` – wencakisa Aug 10 '17 at 07:39
Yeah, agreed. that's somehow same solution from author of tutorial: ^lists/(\d+)/$. But I just wonder about my try. – dvn0zzz Aug 10 '17 at 07:41

score 0 · Answer 3 · answered Aug 10 '17 at 08:09

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You should restrict your pattern to capture only what you want. In your case, it seems to be digits, so:

url(r'^lists/(\d+)/$', views.list_view, name='list_view')

answered Aug 10 '17 at 08:09

Daniel Roseman

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I wonder about my try because if my URL is `lists//` instead of `lists//` then solutions like yours will not work. – dvn0zzz Aug 10 '17 at 08:13
Yes but then you need a different pattern, just as you will need different logic in the view. You should make your patterns as specific as possible. – Daniel Roseman Aug 10 '17 at 08:15
In any case, if you wanted to capture all alphanumeric characters, you would just use `\w+`. – Daniel Roseman Aug 10 '17 at 08:16
let's say: what should be pattern if I want different views for `lists/username/` and `lists/username/add_item`? – dvn0zzz Aug 10 '17 at 08:19
That is exactly what I just said. `r'^lists/(\w+)/$` and `r'^lists/(\w+)/add_item$`. – Daniel Roseman Aug 10 '17 at 08:27

How Django resolve this non-greedy regex url pattern?

3 Answers3