Groups and lookarounds Updated November 2026

Capture groups

Parentheses group part of a pattern and capture what it matched: (\d{4})-(\d{2}) stores the year in group 1 and the month in group 2. Groups let you apply a quantifier to a sequence, (ab)+, reuse the text with $1 in a replacement or \1 in the pattern, and read it from match()[1] in JavaScript.

Syntax

(pattern)      read: match[1], m.group(1)      reuse: $1 in replacements, \1 in the pattern

Examples

"2024-03".match(/(\d{4})-(\d{2})/)

Result: ["2024-03", "2024", "03"]

Index 0 is the whole match, then the groups in order.

"John Smith".replace(/(\w+) (\w+)/, "$2, $1")

Result: "Smith, John"

[..."a=1, b=2".matchAll(/(\w)=(\d)/g)].map(m => [m[1], m[2]])

Result: [["a", "1"], ["b", "2"]]

matchAll gives every match with its groups.

/^(ab)+$/.test("ababab")

Result: true

The group lets + repeat the whole sequence.

Common mistakes

Calling str.match with the g flag and expecting groups; it returns only the full matches. Use matchAll or exec.
Writing \1 in a replacement string; JavaScript replacements use $1, while \1 belongs inside the pattern.
Miscounting groups: they are numbered by the position of the opening parenthesis, including nested ones.
Repeating a group, (\d)+, and expecting all iterations; a group keeps only its last match.

Practice Capture groups

These drills check your answer on live data, free, no account needed.

Capture a standalone yearCapture the first standalone four-digit number on each line in group 1. Digits that are part of a longer number do not count. Lines without one give no match.
Repeated group: dotted numbersThe whole line must match. Match one or more numbers separated by single dots, like 1, 1.2 or 10.0.3. No leading, trailing or doubled dots.
Swap Last, First to First LastLines that are exactly Last, First become First Last. Names contain letters, apostrophes and hyphens. Leave other lines unchanged.

Questions

How do I get a captured group in JavaScript?

const m = str.match(re); m[1] is the first group, or null if there was no match. With the g flag use [...str.matchAll(re)] and read m[1] on each item.

What is group 0?

The whole match. match()[0] and Python's m.group(0) return the full matched text; numbered groups start at 1.

How many capture groups can I have?

Practically unlimited, but $10 and above work only when at least ten groups exist, and large group counts hurt readability. Switch to named groups beyond three or four.

Non-capturing group (?:...)How (?:...) groups for quantifiers and alternation without a numbered capture, why that keeps group numbers stable, and how it changes findall in Python.
Named groups (?<name>...)How to capture by name, read match.groups.name, use $<name> in replacements and \k<name> as a backreference, and the (?P<name>) syntax Python needs.
Backreferences \1What \1 means, finding doubled letters and repeated words, matching a closing quote equal to the opening one, and \1 in a pattern versus $1 in a replacement.
match, test, replace in JavaScriptWhich JavaScript regex method to use for a yes/no check, the first match, all matches with groups, substitution and splitting, with return values and g rules.