Capturing Groups, Named Groups, and Backreferences

Matching text is only half of what regular expressions do. The other half is pulling pieces out of a match and rearranging them, and that is the job of groups. Parentheses turn a regex from a yes-or-no test into a tool that extracts a date's year, swaps the halves of a name, or finds a word repeated by accident. Follow along in the Regex Tester, which shows each captured group as you build the pattern.

Parentheses capture

Wrapping part of a pattern in parentheses does two things at once: it groups that part so a quantifier can apply to all of it, and it captures whatever the part matched so you can refer to it afterwards. Captured groups are numbered from left to right by the position of their opening parenthesis, starting at one. Group zero is always the whole match.

pattern:  (\d{4})-(\d{2})-(\d{2})
text:     2026-06-30

group 1 = 2026   group 2 = 06   group 3 = 30

With those three captures in hand, a replacement string can reorder them. Using $1, $2, $3 to refer to the groups, replacing with $3/$2/$1 turns 2026-06-30 into 30/06/2026. That reorder-on-replace pattern is one of the most useful things regex does.

Non-capturing groups: group without storing

Sometimes you need parentheses only to apply a quantifier, not to capture. A non-capturing group, written with a question mark and colon just inside the parentheses, does exactly that. The pattern (?:ab)+ matches one or more repetitions of ab without filling a capture slot. Using non-capturing groups where you do not need the value keeps your group numbers stable and your intent clear.

Named groups: capture by name

Counting parentheses to find group 3 is fragile. Named groups let you label a capture instead. The syntax puts a name in angle brackets right after the opening parenthesis, and you refer to it by that name rather than a number.

pattern:  (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
replace:  $<day>/$<month>/$<year>

2026-06-30  ->  30/06/2026

Named groups make a pattern self-documenting and a replacement readable: $<day>/$<month>/$<year> says what it does at a glance, while $3/$2/$1 makes a reader count. When a pattern has more than one or two captures, names are worth the few extra characters.

Backreferences: match the same text again

A backreference matches the exact text a group already captured, not just the same pattern. Inside the regex, a backslash and a number refer back to a numbered group; for named groups the form is a backslash, k, and the name in angle brackets. This is how you find structure that must repeat.

\b(\w+)\s+\1\b      finds a doubled word: "the the" matches, "the cat" does not
(?<q>["']).*?\k<q>    matches text in quotes, where the closing quote
                     is the same kind as the opening one

Capture, name, and replace in one place

The Regex Tester lists every numbered and named group it finds and previews a replacement as you edit it, so you can see captures and substitutions update together without leaving your browser. For the precise rules on group syntax and named captures, MDN's reference on groups and backreferences is the authority.