Regular expressions in JavaScript

This chapter follows JSON in JavaScript.

A RegExp describes a pattern; String methods run that pattern against text.

Literals and flags

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/pattern/flags;
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new RegExp("pattern", "flags");

Flag	Meaning
`g`	Global—`exec`/`match` find all
`i`	Case insensitive
`m`	Multiline `^` / `$`
`s`	Dot matches newline (`dotAll`)
`u`	Unicode code points
`d`	Match indices (ES2022)
`y`	Sticky—match only at `lastIndex`

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/foo/gi.test("Foo bar"); // true

Method	Use
`str.match(re)`	First match or all with `g`
`str.matchAll(re)`	Iterator of all matches (requires `g`)
`str.search(re)`	Index of first match or `-1`
`str.replace(re, repl)`	Replace first or all with `g`
`str.replaceAll(re\|str, repl)`	Replace all occurrences
`str.split(re)`	Split on pattern

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"2026-11-01".match(/(\d{4})-(\d{2})-(\d{2})/);
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// ["2026-11-01", "2026", "11", "01", ...]

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const re = /a+/g;
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re.exec("baaard"); // ["aa", index: 1, ...]
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re.test("baa"); // true
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re.lastIndex; // updated when /g or /y

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const re = /(?<year>\d{4})-(?<month>\d{2})/;
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const m = re.exec("2026-11");
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m.groups.year; // "2026"

Lookbehind and lookahead assert context without consuming:

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/(?<=\$)\d+/.exec("price $42"); // ["42"]
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/(?!bad)\w+/.exec("good"); // ["good"]

When building a pattern from arbitrary text, escape metacharacters:

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RegExp.escape("a+b"); // literal safe fragment for `new RegExp(...)`

Prefer RegExp.escape over hand-rolled backslash rules when targeting current engines.

Unicode property escapes \p{Script=Latin} need the u flag.
ReDoS (catastrophic backtracking) is a real risk on user-supplied patterns and huge inputs—cap length or use dedicated parsers for untrusted data.
For JSON text, use JSON.parse, not regex—see the previous chapter.