Groups, Anchors & Lookarounds
Capture parts of a match, reuse them, and assert context with lookarounds.
Groups ( ) capture part of a match so you can extract or reuse it. Captures
are numbered \1, \2 (or $1 in replacements), and can be named
(?P<year>\d{4}). A non-capturing group (?: ) groups without saving.
Lookarounds assert context without consuming characters:
- Lookahead
(?=…)/ negative(?!…)— “followed by” / “not followed by” - Lookbehind
(?<=…)/ negative(?<!…)— “preceded by” / “not preceded by”
These let you match something only when its surroundings qualify — e.g. a number followed by “USD” without including “USD” in the match.
import re
m = re.search(r"(?P<year>\d{4})-(?P<month>\d{2})", "2026-06")
print(m.group("year"), m.group("month")) # 2026 06
# Replace using captured groups: swap "First Last" -> "Last, First"
print(re.sub(r"(\w+)\s+(\w+)", r"\2, \1", "Ada Lovelace")) # Lovelace, Ada
# Lookahead: amounts followed by USD (without matching USD)
print(re.findall(r"\d+(?= USD)", "10 USD, 5 EUR, 20 USD")) # ['10', '20']- Capture the year and month from a date string into named groups.
- Use backreferences to reformat “First Last” into “Last, First”.
- Write a lookahead that matches a price only when followed by a currency code.
- Explain the difference between
(abc)and(?:abc).
Cheat Sheet▾
| Token | Meaning |
|---|---|
(…) | capturing group |
(?:…) | non-capturing group |
(?P<name>…) | named group |
\1 / $1 | backreference |
(?=…) / (?!…) | lookahead / negative |
(?<=…) / (?<!…) | lookbehind / negative |
Common Interview Questions▾
What is a capturing group used for?
To extract or reuse part of a match — via group numbers/names for extraction, or backreferences (\1, $1) in replacements and within the pattern.
What does a lookahead do?
It asserts that a position is (or isn’t) followed by a pattern without consuming those characters, so they stay out of the match itself.