Using Python group()

1. Introduction

In Python, group() is a method in regular expressions. It takes a string that matches a regular expression and returns a tuple containing all matching substrings. This method finds all substrings in a string that match a regular expression and returns them as a tuple.

2. Syntax

group() has the following syntax:

re_object.group([group1, ...])

The group1, ... parameters are optional. If no parameters are specified, the group() method returns the entire matched string. If one or more parameters are specified, the method returns the matched string for the groups corresponding to those parameters.

3. Examples

Next, we’ll use some examples to demonstrate the use of the group() method.

Example 1: Returning the Entire Matched String

import re

sentence = 'I have 2 cats and 3 dogs.'

pattern = r'[0-9]+'

match = re.search(pattern, sentence)

print(match.group())

Running Result:

2

In this example, we define a string sentence and a matching pattern pattern. We use the search() method of the re module to find the first match 2 in the string, and then use the group() method to retrieve the entire matched string 2.

Example 2: Return the matching string for a specified group

import re

sentence = 'I have 2 cats and 3 dogs.'

pattern = r'I have ([0-9]+) cats and ([0-9]+) dogs.'

match = re.search(pattern, sentence)

print(match.group(1))
print(match.group(2))

Running result:

2
3

In this example, we define a more complex matching pattern pattern, which contains two groups: the first group matches the number of cats, and the second group matches the number of dogs. We used the search() method of the re module to find the first matches of 2 and 3 in the string, and then used the group() method to retrieve the two matching strings.

4. Notes

• If the regular expression does not use parentheses to group the string, the group() method will only return the entire matching string.
• The string returned by the group() method is immutable.
• If a group does not match the string, the corresponding group() method will return None.
• If the specified group does not exist, the corresponding group() method will throw an IndexError exception.

5. Summary

This article introduced the use of the group() method in Python, a regular expression method used to retrieve strings that match a regular expression. We demonstrated the use of the group() method through examples and explained some precautions.