[Django]-What does this Django regular expression mean? `?P`

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.

(?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/

New in version 3.6.

(?P<name>...)
Similar to regular parentheses, but the substring matched by the group is accessible 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.

copy paste from Python3Regex

In pattern matching,
Use this pattern for passing string

(?P<username2>[-\w]+)

This for interger value

(?P<user_id>[0-9]+)