Patterns for searching, validating and transforming text. Write them as literals /pattern/flags or with new RegExp(string, flags).

Syntax cheat sheet

Token	Matches
`.`	Any char except newline (any char with the `s` flag)
`\d \w \s`	Digit, word char `[A-Za-z0-9_]`, whitespace — uppercase negates
`[abc]`, `[^abc]`, `[a-z]`	Character class, negated class, range
`^` `$` `\b`	Start, end (per line with `m`), word boundary
`* + ? {n,m}`	0+, 1+, 0–1, n to m — add `?` for lazy (`*?`)
`(…)` `(?<name>…)` `(?:…)`	Capture group, named group, non-capturing group
`a\|b`	Alternation
`(?=…)` `(?!…)` `(?<=…)` `(?<!…)`	Lookahead, negative lookahead, lookbehind, negative lookbehind

Flag	Meaning
`g`	Global — all matches
`i`	Case-insensitive
`m`	Multiline `^`/`$`
`s`	`.` matches newlines (dotAll)
`u` / `v`	Unicode mode / Unicode sets (ES2024)
`y`	Sticky — match at `lastIndex` only
`d`	Record match indices

The main methods

JavaScript

const text = "Order #123 shipped on 2024-03-15, order #456 on 2024-04-01.";
console.log(/order/i.test(text));
console.log(text.match(/#\d+/));              // first match + details
console.log(text.match(/#\d+/g));             // all matches (no details)
console.log(text.search(/\d{4}/));
console.log(text.replace(/(\d{4})-(\d{2})-(\d{2})/g, "$3/$2/$1"));
console.log("a1b22c333".split(/\d+/));

Output

true
[
  '#123',
  index: 6,
  input: 'Order #123 shipped on 2024-03-15, order #456 on 2024-04-01.',
  groups: undefined
]
[ '#123', '#456' ]
22
Order #123 shipped on 15/03/2024, order #456 on 01/04/2024.
[ 'a', 'b', 'c', '' ]

Named groups and matchAll

JavaScript

const log = "user=asha id=7; user=ravi id=12";
const re = /user=(?<name>\w+) id=(?<id>\d+)/g;
for (const m of log.matchAll(re)) {
  console.log(m.groups.name, Number(m.groups.id), m.index);
}
const { year, month } = "2024-09".match(/(?<year>\d{4})-(?<month>\d{2})/).groups;
console.log(year, month);

Output

asha 7 0
ravi 12 16
2024 09

Greedy vs lazy, lookarounds

JavaScript

const html = "<b>bold</b> and <i>italic</i>";
console.log(html.match(/<.+>/)[0]);       // greedy: as much as possible
console.log(html.match(/<.+?>/)[0]);      // lazy: as little as possible
console.log("1234567.89".replace(/\B(?=(\d{3})+(?!\d))/g, ","));
console.log("price: $30, €40".match(/(?<=\$)\d+/)[0]);
const strong = /^(?=.*[A-Z])(?=.*\d)(?=.*[^\w\s]).{8,}$/;
console.log(strong.test("Passw0rd!"), strong.test("password"));

Output

<b>bold</b> and <i>italic</i>
<b>
1,234,567.89
30
true false

The `lastIndex` trap

JavaScript

const re = /a/g;
console.log(re.test("a"), re.test("a"), re.test("a"));

Output

true false true

Dynamic patterns

JavaScript

const escapeRegExp = (s) => s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
const term = "a.b";
console.log(new RegExp(term).test("axb"), new RegExp(escapeRegExp(term)).test("axb"));
console.log(new RegExp(`^${escapeRegExp("1+1")}$`).source);

Output

true false
^1\+1$

What does the g flag change?

Syntax cheat sheet

The main methods

Named groups and matchAll

Greedy vs lazy, lookarounds

The lastIndex trap

Dynamic patterns

The `lastIndex` trap