15 — Regular expressions

One line: A JavaScript regex carries state as well as a pattern: its flags change what . and \p{…} mean, and with g the same call can answer differently the second time.

The chapter begins with the literal and its eight flags, then the state that g and y keep in lastIndex, which explains most surprises in the three search methods that follow. Named groups and the replacement mini-language come next, then Unicode mode, where u and v change what . matches. It ends where JavaScript differs from other engines: lookbehind of any length, and RegExp.escape. The Regex library asks the same questions across eight engines.

The pages

Page	Level	In one line	Status
Regex literals and flags	101	A regex has eight flags, and .flags reports them in one fixed order whatever order you typed — /a/yigd.flags is "dgiy" — while u with v, a repeated flag or an unknown letter is a SyntaxError.	stub
The g flag and lastIndex	201	A regex with g or y remembers where its last match ended in lastIndex, so three test("a") calls on one /a/g answer true, false, true, and a shared /c/g filtering ["cat", "car", "cow"] drops "car".	stub
match, matchAll and exec	201	"a1 b2".match(/([a-z])(\d)/g) returns only ["a1", "b2"] — with g, match drops the groups — while matchAll keeps them but throws a TypeError without g, and a failed match returns null, not an empty array.	stub
Named groups and replacements	201	The replacement string is a small language of its own: "$<d>.$<m>.$<y>" reorders named groups and $& inserts the whole match, so user text "$& off" turns "price" into "price off", while a replacer function's return value goes in literally.	stub
Unicode mode	301	Without u, . matches one UTF-16 code unit, so /^.$/.test("😀") is false and /^..$/ is true; u makes . a code point, yet 👨‍👩‍👧 still takes five dots, and v adds \p{RGI_Emoji}, which matches it as one.	stub
Lookbehind	201	JavaScript accepts a lookbehind of any length — (?<=\$\d*)\d finds every digit of a price — and matches it right to left, so /(?<=(\d+)(\d+))$/.exec("1053") captures "1" and "053", while the same groups matched forwards give "105" and "3".	stub
RegExp.escape	201	RegExp.escape (ES2025, in Node 24) turns text into a pattern that matches only itself — new RegExp("1.5") matches "125", the escaped one does not — and it escapes more than punctuation: RegExp.escape("1.5") is \x31\.5.	stub

Boundaries

Pattern syntax shared by every engine — quantifiers, classes, backtracking — is covered in the Regex library, which has a JavaScript column on most pages.