exec()?RegExp.prototype.exec(string) runs a search and returns match details (full match, groups, position) or null if nothing matches. It is the detailed cousin of test().
regex.exec(string)
match found → array:
[0] full match
[1…] capture groups
.index start position
.input original string
no match → null
with /g: each call advances lastIndex| Use | Prefer |
|---|---|
| Match + groups + position | regex.exec(str) |
| Yes / no only | regex.test(str) |
| All matches (global) | Loop exec() until null |
exec()Create a regex, call exec, then read the result or handle null.
new RegExp).regex.exec(string).null, read [0], groups, and .index.g flag, call exec again in a loop until it returns null.| Piece | Required? | Role |
|---|---|---|
| RegExp | Yes | Pattern to search with |
string | Yes | Text to search in |
| Null check | Yes in practice | Avoid reading properties on null |
regex = /pattern/flags
str = text to search
result = regex.exec(str)
if result is null:
// no match
else:
full = result[0]
group1 = result[1] // if any
pos = result.index
// with /g flag:
while (result = regex.exec(str)) is not null:
use result
// lastIndex advances each timeKeep these facts handy while reading match results.
| Topic | Detail |
|---|---|
| Syntax | regex.exec(string) |
| Success | Array with match + optional groups |
| Failure | null |
result.index | Where the match starts |
result.input | Original string |
Global /g | Uses / updates lastIndex |
vs test() | test → boolean; exec → details |
Pick a pattern and see what exec() returns on sample text.
Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.
Get the matched text and its start index.
const regex = /cat/i;
const result = regex.exec("The Cat sat");
console.log(result[0]); // "Cat"
console.log(result.index); // 4
console.log(/dog/.exec("The Cat sat")); // null/cat/i matches cat ignoring case."Cat" inside "The Cat sat".result[0] is the full match; index is 4./dog/ finds nothing → null.Read parts of the match with parentheses.
const regex = /(\w+)-(\d+)/;
const result = regex.exec("Code-42");
console.log(result[0]); // "Code-42"
console.log(result[1]); // "Code"
console.log(result[2]); // "42"(\w+) and (\d+) capture word and digits.result[0] is the entire "Code-42"."Code"."42".lastIndexCollect every match using the g flag.
const regex = /\d+/g;
const str = "A12 B34 C5";
let match;
while ((match = regex.exec(str)) !== null) {
console.log(match[0] + " @ " + match.index);
}
// "12 @ 1"
// "34 @ 5"
// "5 @ 9"/\d+/g can find more than one number.lastIndex.exec returns null.Check these before relying on exec() in real projects.
nullReading result[0] when exec returned null throws. Guard with if (result).
With /g, reuse can continue from a stale lastIndex. Reset with regex.lastIndex = 0 when restarting a search.
Patterns that can match empty strings (e.g. /a*/g on some inputs) need care — advance lastIndex manually if a match has length 0.
test() also updates lastIndex when globalDo not mix test and exec on the same global regex without resetting lastIndex.
RegExp.prototype.exec() is a long-standing core method — available in every JavaScript environment.
Supported in all browsers and Node.js versions. Safe for every target with no polyfill.
Bottom line: Use freely for match details and capture groups. Prefer test() when you only need a boolean.
| Approach | Cost | Best for |
|---|---|---|
regex.exec(str) | One search step | Details + groups |
regex.test(str) | One search step | Boolean only (lighter intent) |
Global exec loop | One step per match | All matches with groups |
Prefer test() when you do not need groups or position. For many matches with captures, a global exec loop is the classic approach.
null), including groups and index.null before reading the result.exec() with /g until null; watch lastIndex.test() for yes/no; exec() for details.One line: exec() gives you the match, groups, and position — or null when nothing matches.
Learn how the g flag and lastIndex change matching behavior.
10 people found this page helpful