Regular Expression Flags

Flags are optional modifiers that change the behavior of a regular expression. They are added after the closing / in literal syntax or as the second argument in the RegExp constructor.

1. Global Flag (g)

Searches for all matches in a string, not just the first one.

• Often used with replace(), match(), and loops.

const str = "apple banana apple";
const regex = /apple/g;

console.log(str.match(regex)); // ["apple", "apple"]
console.log(str.replace(regex, "orange")); // "orange banana orange"

2. Case-Insensitive Flag (i)

Makes the match case-insensitive.

const str = "Hello World";
const regex = /hello/i;

console.log(regex.test(str)); // true

3. Multiline Flag (m)

Changes the behavior of ^ (start) and $ (end) to match the start/end of each line, not just the string.

const str = `Hello
World`;

const regex1 = /^World$/;
console.log(regex1.test(str)); // false

const regex2 = /^World$/m;
console.log(regex2.test(str)); // true

4. Dot-All Flag (s)

Makes the dot . match newline characters as well.

• Useful for matching multi-line content.

const str = "Hello\\nWorld";
const regex1 = /Hello.World/;
console.log(regex1.test(str)); // false

const regex2 = /Hello.World/s;
console.log(regex2.test(str)); // true

5. Unicode Flag (u)

Enables full Unicode support, including emojis and special characters.

• Required when working with Unicode code points > 0xFFFF.

const str = "?";
const regex = /\\u{1F496}/u;

console.log(regex.test(str)); // true

6. Sticky Flag (y)

Matches from the exact position in the string (like lastIndex) rather than searching ahead.

• Often used in tokenizers and parsers.

const str = "hello hello";
const regex = /hello/y;

console.log(regex.exec(str)); // ["hello"]
regex.lastIndex = 6;
console.log(regex.exec(str)); // ["hello"]

Summary Table

Flag	Description
g	Global search (all matches)
i	Case-insensitive search
m	Multiline mode (^ and $ match each line)
s	Dot matches newline (. matches \\n)
u	Unicode mode
y	Sticky search (match from lastIndex)