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:

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The following regular expression matches the above path:

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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':

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Output:

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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:

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

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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:

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Output:

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

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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:

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Output:

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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:

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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:

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Output:

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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:

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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:

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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:

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Output:

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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:

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matches this path:

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

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

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Output:

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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.