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I have the following regular expression (regex) in my urls.py and I'd like to know what it means. Specifically the (?P<category_slug> portion of the regex.
r'^category/(?P<category_slug>[-\w]+)/$ 
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The P is Python identifier for a named capture group. You will see P in regex used in jdango and other python based regex implementations.
$ means "Match the end of the string" (the position after the last character in the string).
P stands for Placeholder . – kev. Apr 8, 2012 at 1:20.
Basically (0+1)* mathes any sequence of ones and zeroes. So, in your example (0+1)*1(0+1)* should match any sequence that has 1. It would not match 000 , but it would match 010 , 1 , 111 etc. (0+1) means 0 OR 1.
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.
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(?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
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