Match 1
characters 0–6Ada 37Group $1[0, 3)
AdaGroup $2[4, 6)
37Named groups
name[0, 3)
AdaInteractive capture-group debugger
Regex Capture Group Tester & Explorer
Paste a JavaScript regular expression and test text to inspect exactly what each capture group contains. See full matches, numbered captures, named groups, and character ranges without sending your input to a server.
//
Try flags such as g, i, m, s, or u. With no g or y, JavaScript returns only the first match.
Live result
Captured groups
Match 2
characters 7–15Linus 55Group $1[7, 12)
LinusGroup $2[13, 15)
55Named groups
name[7, 12)
LinusMatch 3
characters 16–24Grace 85Group $1[16, 21)
GraceGroup $2[22, 24)
85Named groups
name[16, 21)
GraceNumbered vs named capture groups
Parentheses such as (\\d+) create numbered captures in left-to-right opening-parenthesis order. JavaScript named groups use (?<name>...), which lets application code read match.groups.name instead of relying only on a numeric position.
/(?<year>\d{4})-(\d{2})-(\d{2})/
// full match: 2026-08-17
// $1 / year: 2026
// $2: 08
// $3: 17When not to capture
Use (?:...) when parentheses exist only to control precedence or apply a quantifier. Non-capturing groups keep the result easier to reason about and prevent later edits from unexpectedly shifting numbered group positions.
If a pattern is difficult to understand even with this explorer, move to the Regex Debugger or test the full expression in the Regex Tester.
Common capture-group debugging checks
Optional groups
A group inside ? or an unmatched alternative may legitimately be undefined for a particular match.
Group numbering
Nested capturing parentheses still consume group numbers. Convert structure-only parentheses to (?:...) when captures are not needed.
Global matching
The g flag returns repeated matches. Each match has its own capture values, so inspect groups per match rather than assuming one global value.
Frequently asked questions
What is a capture group in regex?
A capture group is a parenthesized part of a regular expression whose matched text is stored separately from the full match. Numbered groups use parentheses such as (\d+), while named groups use syntax such as (?<year>\d{4}) in JavaScript.
What is the difference between a capture group and a non-capturing group?
A capturing group stores its matched substring for later inspection or backreferences. A non-capturing group, written (?:...), groups alternatives or quantifiers without creating a numbered capture.
Why is a capture group undefined?
Optional groups and alternatives do not have to participate in every match. When a group does not participate, JavaScript returns undefined for that capture.
Does this explorer use JavaScript regex behavior?
Yes. Patterns are evaluated with the browser JavaScript RegExp engine, so group syntax, flags, lookbehind support, Unicode behavior, and backreferences follow ECMAScript semantics.