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.

RegExp.prototype.global • g

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

Find all matches.

Used by some regex functions. e.g. String.prototype.match • String.prototype.replace

// 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 regexobj = /([aeiou])/g;
console.assert(regexobj.global === true);

RegExp.prototype.ignoreCase • i

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

Ignore case.

// demo diff of ignoreCase

console.assert(/a/i.test("ABC"));

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

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

RegExp.prototype.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.assert(/^B/.test("A\nB") === false);
console.assert(/^B/m.test("A\nB"));

console.assert(/a/m.multiline);

RegExp.prototype.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.assert(/A.B/.test("A\nB") === false);
console.assert(/A.B/s.test("A\nB"));

RegExp.prototype.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)

RegExp.prototype.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