What does this Django regular expression mean? `?P`

(?P<category_slug>) creates a match group named category_slug.

The regex itself matches a string starting with category/ and then a mix of alphanumeric characters, the dash - and the underscore _, followed by a trailing slash.

Example URLs accepted by the regex:

• category/foo/
• category/foo_bar-baz/
• category/12345/
• category/q1e2_asdf/

(?P<name>regex) - Round brackets group the regex between them. They capture the text matched by the regex inside them that can be referenced by the name between the sharp brackets. The name may consist of letters and digits.

Copy paste from: http://www.regular-expressions.info/refext.html

In django, named capturing groups are passed to your view as keyword arguments.

Unnamed capturing groups (just a parenthesis) are passed to your view as arguments.

The ?P is a named capturing group, as opposed to an unnamed capturing group.

http://docs.python.org/library/re.html

(?P<name>...) Similar to regular parentheses, but the substring matched by the group is accessible within the rest of the regular expression via the symbolic group name name. Group names must be valid Python identifiers, and each group name must be defined only once within a regular expression. A symbolic group is also a numbered group, just as if the group were not named. So the group named id in the example below can also be referenced as the numbered group 1.