JS: Regular Expression Flags

What is Regex Flag

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

List of Regex Flags

global (g)

• Property Name: RegExp.prototype.global
• Short Code: g

Find all matches.

Used by many regex functions. 〔see JS: Regular Expression Functions〕

// demo diff of regex flag g

/* get all occurrence of a match */
console.log("something".match(/[aeiou]/g));
// [ "o", "e", "i" ]

console.log("something".match(/[aeiou]/));
// [ "o", index: 1, input: "something", groups: undefined ]

// RegExp.prototype.global
const xx = /([aeiou])/g;
console.log(xx.global);
// true

ignoreCase (i)

• Property Name: RegExp.prototype.ignoreCase
• Short Code: i

Ignore case.

// demo diff of ignoreCase

console.log(/a/i.test("ABC") === true);

console.log(/a/.test("ABC") === false);

const xx = /a/i;
console.log(xx.ignoreCase);
// true

multiline (m)

• Property Name: RegExp.prototype.multiline
• Short Code: m

• Make the regex syntax ^ match any newline beginning. (instead of the beginning of whole string)
• Make the regex 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);

console.log(/a/m.multiline);
// true

dotAll (s)

(new in ECMAScript 2018)

• Property Name: RegExp.prototype.dotAll
• Short Code: s

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);

unicode (u)

(new in ECMAScript 2015)

• Property Name: RegExp.prototype.unicode
• Short Code: u

Interpret the text to be matched as byte sequence of characters of their unicode code points.

• JS: RegExp.prototype.unicode (flag u)

sticky (y)

(new in ECMAScript 2015)

• Property Name: RegExp.prototype.sticky
• Short Code: y

Make the match start at the index RegExp.prototype.lastIndex. (does not change the meaning of ^. It still mean beginning of string or line.)

• JS: RegExp.prototype.lastIndex