JS: RegExp Flag

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

RegExp Flags

g (State stored in RegExp.prototype.global)

Find all matches.

Used by many Regex Functions

i (State stored in RegExp.prototype.ignoreCase)

Ignore case.

m (State stored in RegExp.prototype.multiline)

• Make the RegExp syntax ^ match any newline beginning. (instead of the beginning of whole string)
• Make the RegExp syntax $ match any newline end. (instead of the end of whole string)

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

u (State stored in RegExp.prototype.unicode)

(JS2015) Treat string as sequence of unicode characters. (as opposed to being JS: String Code Unit)

💡 TIP: It's a good idea to always have this flag on.

💡 TIP: this flag is useful to prevent matching a surrogate pair code unit in input string. 〔see JS: String Code Unit〕 It's not for “enabling” matching Unicode. (Unicode characters are matched always.)

💡 TIP: To match Unicode character properties, use unicode propery character class syntax. 〔see RegExp Unicode Property〕

console.log(/\ud83d/.test("😂") === true);
console.log(/\ud83d/u.test("😂") === false);

/*

U+D83D is not a valid unicode character.
D83D is hexadecimal of first code unit of a surrogate pair for 😂

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.

😂
FACE WITH TEARS OF JOY
codepoint 128514
codepoint in hexadecimal 1f602
bytes in UTF-16: D8 3D DE 02

 */

y (State stored in RegExp.prototype.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.)

s (State stored in RegExp.prototype.dotAll)

(JS2018) Make the dot . also match newline characters. 〔see RegExp Syntax〕

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