Groups and lookarounds Updated November 2026

Backreferences \1

A backreference \1 matches the same text that group 1 captured earlier in the pattern: (\w)\1 finds a doubled letter and \b(\w+) \1\b finds a repeated word. In a replacement string use $1 (JavaScript) or \1 (Python); \k<name> refers back to a named group.

Syntax

(x)\1      \k<name>      (["']).*?\1

Examples

"the the cat sat sat".match(/\b(\w+) \1\b/g)

Result: ["the the", "sat sat"]

"bookkeeper".match(/(\w)\1/g)

Result: ["oo", "kk", "ee"]

"the the cat".replace(/\b(\w+) \1\b/g, "$1")

Result: "the cat"

Collapse the repeated word; the replacement uses $1, not \1.

['"ok"', '"bad\''].map(s => /^(["']).*\1$/.test(s))

Result: [true, false]

The closing quote must equal the opening one.

Common mistakes

Writing \1 in a JavaScript replacement string; it inserts a literal backslash-1. Use $1.
Referring to a group before it has captured, such as \1(a); the reference is empty or fails.
Relying on \1 across engines when group 1 did not participate: JavaScript matches the empty string, most other engines fail the match.
Ambiguity with \10: it means group 10 if that many groups exist, otherwise group 1 followed by a 0.

Practice Backreferences \1

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

Backreference: doubled wordsA match anywhere counts. Match lines where the same whole word appears twice in a row, separated by one space, like the the.
Matching quote marksThe whole line must match. Match text wrapped in quotes, where the closing quote is the same kind as the opening one: both double or both single.
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.

Questions

What is \1 in regex?

A reference to whatever capture group 1 matched, so the same text must appear again at that point. \2 refers to group 2, and so on.

How do I find duplicate words with regex?

/\b(\w+)\s+\1\b/gi finds a word followed by itself, case-insensitively, with any whitespace between (the the, The the).

Can a backreference match across lines?

Yes. The captured text is compared wherever the reference sits; only the dot and anchors care about line breaks. Use \s+ instead of a space to allow newlines between repeats.

Capture groupsHow parentheses capture text, reading groups in JavaScript via match, matchAll and exec, reusing them with $1 in replace, and the g trap that loses groups.
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.
Word boundary \bWhat \b matches, how to find or replace whole words only, what \B does, and why \b fails next to punctuation, in string-built regexes and with Unicode text.
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.