Capture

import re

line = 'There is a phone number 12345 in this row and an age: 23'

match = re.search(r'age: \d+', line)
if match:
  print(match.group(0))  # age: 23


match = re.search(r'age: (\d+)', line)
if match:
    print(match.group(0))  # age: 23
    print(match.group(1))  # 23      the first group of parentheses

    print(match.groups())  # ('23',)
    print(len(match.groups()))  # 1

Given that string we would like to extract the number that comes after the age:.

The first example shows a regular expression age: \d+ that will match the characters age: , with the trailing space and the one or more digits. That works and if we check the content of group(0) we'll see that it indeed matched what we asked for age: 23, hower that's not exactly what we wanted. We only wanted the number.

So in the second example we put a pair of parentheses in the regular expression around a sub-expression: age: (\d+). This does not impact the matching and group(0) still returns the whole substring that was matched, but now we can also call group(1) that will return whatever the sub-expression inside the parentheses matched.

Whatever this sub-expression matches will be saved and can be accessed using the group() method.

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