JavaScript: RegExp Flags

By Xah Lee. Date: . Last updated: .

Regex flags change the meaning of the regular expression, or behavior of Regex Functions . For how to add them, see JavaScript: RegExp Syntax

RegExp Flags

g (aka global)
Find all matches. Don't stop after first found. Used by:

State stored in

i (aka ignoreCase)
Ignore case. Example: "WATER".search( /t/i ).

State stored in RegExp.prototype.ignoreCase.

m (aka multiline)
Make the RegExp syntax ^ and $ match any newline beginning/end. (instead of the beginning/end of whole string)

State stored in RegExp.prototype.multiline.

console.log(/^B/.test("A\nB") === false);
console.log(/^B/m.test("A\nB") === true);
u (aka unicode)
(JS2015) Treat string as sequence of unicode characters. If your string contains a character whose codepoint is β‰₯ 2^16, such as emoji, you shoud set this flag. It's a good idea to always have this flag on.

(Tip: this flag is useful to prevent matching a code unit in input string. [see JavaScript: String Code Unit] It's not for β€œenabling” matching Unicode. (Unicode characters are matched always.) To match Unicode character properties, use unicode propery character class syntax. [see RegExp Unicode Property] )

State stored in RegExp.prototype.unicode.

console.log(/\ud83d/u.test("πŸ˜‚") === false);

// U+D83D is not a valid unicode character. D83D is hexadecimal of first code unit of a surrogate pair for πŸ˜‚

/* [
codepoint 128514
codepoint in hexadecimal 1f602
bytes in UTF-16: D8 3D DE 02
So in JavaScript , the char is made up of 2 code units: D83D DE02
When you search for D83D without unicode flag, it returns true.
but with unicode flag, it returns false.
] */
y (aka sticky)
(JS2015) Make the match start at the index RegExp.prototype.lastIndex. (does not change the meaning of ^. It still mean beginning of string or line.)

State stored in RegExp.prototype.sticky.

s (aka dotAll)
(JS2018) Make the dot . also match newline characters. [see RegExp Syntax]

State stored in RegExp.prototype.dotAll.

console.log(/A.B/.test("A\nB") === false);
console.log(/A.B/s.test("A\nB") === true);

JavaScript Regular Expression

JavaScript in Depth

JS Obj Reference