Python Regex Capturing Group

Summary: in this tutorial, you’ll learn about Python regex capturing groups to create subgroups for a match.

Introduction to the Python regex capturing groups

Suppose you have the following path that shows the news with the id 100 on a website:

news/100
Code language: Python (python)

The following regular expression matches the above path:

\w+/\d+
Code language: Python (python)

Note that the above regular expression also matches any path that starts with one or more word characters, e.g., posts, todos, etc. not just news.

In this pattern:

  • \w+ is a word character set with a quantifier (+) that matches one or more word characters.
  • / mathes the forward slash / character.
  • \d+ is digit character set with a quantfifer (+) that matches one or more digits.

The following program uses the \w+/\d+ pattern to match the string ‘news/100':

import re s = 'news/100' pattern = '\w+/\d+' matches = re.finditer(pattern,s) for match in matches: print(match)
Code language: Python (python)

Output:

<re.Match object; span=(0, 8), match='news/100'>
Code language: Python (python)

It shows one match as expected.

To get the id from the path, you use a capturing group. To define a capturing group for a pattern, you place the rule in parentheses:

(rule)
Code language: Python (python)

For example, to create a capturing group that captures the id from the path, you use the following pattern:

'\w+/(\d+)'
Code language: Python (python)

In this pattern, we place the rule \d+ inside the parentheses (). If you run the program with the new pattern, you’ll see that it displays one match:

import re s = 'news/100' pattern = '\w+/(\d+)' matches = re.finditer(pattern, s) for match in matches: print(match)
Code language: Python (python)

Output:

<re.Match object; span=(0, 8), match='news/100'>
Code language: Python (python)

To get the capturing groups from a match, you the group() method of the Match object:

match.group(index)
Code language: Python (python)

The group(0) will return the entire match while the group(1), group(2), etc., return the first, second, … group.

The lastindex property of the Match object returns the last index of all subgroups. The following program shows the entire match (group(0)) and all the subgroups:

import re s = 'news/100' pattern = '\w+/(\d+)' matches = re.finditer(pattern, s) for match in matches: for index in range(0, match.lastindex + 1): print(match.group(index))
Code language: Python (python)

Output:

news/100 100
Code language: Python (python)

In the output, the news/100 is the entire match while 100 is the subgroup.

If you want to capture also the resource (news) in the path (news/100), you can create an additional capturing group like this:

'(\w+)/(\d+)'
Code language: Python (python)

In this pattern, we have two capturing groups one for \w+ and the other for \d+ . The following program shows the entire match and all the subgroups:

import re s = 'news/100' pattern = '(\w+)/(\d+)' matches = re.finditer(pattern, s) for match in matches: for index in range(0, match.lastindex + 1): print(match.group(index))
Code language: Python (python)

Output:

news/100 news 100
Code language: Python (python)

In the output, the news/100 is the entire match while news and 100 are the subgroups.

Named capturing groups

By default, you can access a subgroup in a match using an index, for example, match.group(1). Sometimes, accessing a subgroup by a meaningful name is more convenient.

You use the named capturing group to assign a name to a group. The following shows the syntax for assigning a name to a capturing group:

(?P<name>rule)
Code language: Python (python)

In this syntax:

  • () indicates a capturing group.
  • ?P<name> specifies the name of the capturing group.
  • rule is a rule in the pattern.

For example, the following creates the names:

'(?P<resource>\w+)/(?P<id>\d+)'
Code language: Python (python)

In this syntax, the resource is the name for the first capturing group and the id is the name for the second capturing group.

To get all the named subgroups of a match, you use the groupdict() method of the Match object. For example:

import re s = 'news/100' pattern = '(?P<resource>\w+)/(?P<id>\d+)' matches = re.finditer(pattern, s) for match in matches: print(match.groupdict())
Code language: Python (python)

Output:

{'resource': 'news', 'id': '100'}
Code language: Python (python)

In this example, the groupdict() method returns a dictionary where the keys are group names and values are the subgroups.

More named capturing group example

The following pattern:

\w+/d{4}/d{2}/d{2}
Code language: Python (python)

matches this path:

news/2021/12/31
Code language: Python (python)

And you can add the named capturing groups to the pattern like this:

'(?P<resource>\w+)/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})'
Code language: Python (python)

This program uses the patterns to match the path and shows all the subgroups:

import re s = 'news/2021/12/31' pattern = '(?P<resource>\w+)/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})' matches = re.finditer(pattern, s) for match in matches: print(match.groupdict())
Code language: Python (python)

Output:

{'resource': 'news', 'year': '2021', 'month': '12', 'day': '31'}
Code language: Python (python)

Summary

  • Place a rule of a pattern inside parentheses () to create a capturing group.
  • Use the group() method of the Match object to get the subgroup by an index.
  • Use the (?P<name>rule) to create a named capturing group for the rule in a pattern.
  • Use the groupdict() method of the Match object to get the named subgroups as a dictionary.
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