Grouping with Parentheses

Parentheses scope a piece of pattern

You've already seen parentheses doing scope work in earlier tracks. (ab)+ repeats 'ab' three times; (abc|def) matches either 'abc' or 'def'. The parentheses don't change what's matched — they just tell the engine "this is one thing."

Four jobs in one syntax

Parentheses do four distinct things, all packed into the same syntax:

Scope a quantifier or alternation — (ab)+, (abc|def)
Capture a value — (\d{4})-(\d{2}) captures year and month into groups 1 and 2
Apply lookaround / atomic / etc. — special group syntax like (?=...), (?:...), (?>...)
Provide a target for backreferences — (\w+) \1 matches "word, then the same word again"

This is why parentheses are such workhorses — and why "do I need a capturing group here, or just grouping?" is one of the most common decisions you'll make.

Default is capturing

Plain (...) is a CAPTURING group. The matched substring is allocated a slot you can reference later (group 1, group 2, etc.). If you only need the grouping for scoping (no backreference, no extraction), use the explicitly non-capturing (?:...) covered in lesson 4.

Code

Parentheses doing different jobs·python

import re

# Scoping a quantifier — repeat 'ab' three or more times
re.findall(r'(ab){3,}', 'ababab abababab abc')
# ['ab', 'ab']  — captures last iteration of group

# Scoping an alternation — match 'cat' or 'dog'
re.findall(r'(cat|dog)s?', 'cats and dogs and cat-dog')
# ['cat', 'dog', 'cat', 'dog']

# Capturing a value
m = re.match(r'(\d{4})-(\d{2})', '2026-05-04')
m.group(1), m.group(2)
# ('2026', '05')

# Backreference target
bool(re.search(r'(\w+) \1', 'the the cat sat'))
# True  — 'the' followed by 'the'

Grouping with Parentheses

Parentheses scope a piece of pattern

Four jobs in one syntax

Default is capturing

Code

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Exercise

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