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JS RegExp Flags

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JavaScript RegExp Flags (Modifiers)

In JavaScript Regular Expressions, flags (also known as modifiers) are used to change the default searching behavior. By default, a regex search stops after the very first match it finds, and it is strictly case-sensitive.

Flags allow you to perform global searches, case-insensitive searches, and more. You add flags directly after the closing slash of a regular expression (e.g., /pattern/g).

Common RegExp Flags

Here is a table of the most commonly used flags in JavaScript:

Flag Name Description
i Ignore Case Makes the search case-insensitive. (A matches a)
g Global Performs a global match. Finds all matches rather than stopping after the first match.
m Multiline Makes the start (^) and end ($) boundary assertions match the beginning and end of each line rather than the whole string.
s DotAll Allows the dot . metacharacter to match newline characters (\n).
u Unicode Enables full Unicode matching (useful for emojis and extended characters).
y Sticky Forces the search to only look at the exact lastIndex position in the string.

1. The i Flag (Case-Insensitive)

By default, regular expressions are case-sensitive. The i flag tells the RegExp engine to ignore the casing.

The `i` Flag Example

let text = "Visit LONDON!";


// Without 'i' flag: returns -1 (Not found because of capital L)
console.log(text.search(/london/)); 


// With 'i' flag: returns 6 (Match found!)
console.log(text.search(/london/i));

2. The g Flag (Global Match)

By default, methods like replace() or match() will only process the first occurrence of a pattern. If you want to find or replace all occurrences in a string, you must use the g flag.

The `g` Flag Example

let str = "The rain in SPAIN stays mainly in the plain"; 


// Replace without 'g' (Only replaces the first "in")
console.log(str.replace(/in/, "XX"));


// Replace with 'g' (Replaces ALL occurrences of "in")
console.log(str.replace(/in/g, "XX"));

3. Combining Flags

You can combine multiple flags together to compound their effects! Just place them next to each other after the closing slash. Order does not matter (/gi is the same as /ig).

Combining `g` and `i` Flags

let str = "The rain in SPAIN stays mainly in the plain"; 


// We want to replace ALL "in", ignoring case!
let result = str.replace(/in/gi, "XX");


console.log(result);

Exercise

?

Which flag tells the RegExp engine to find all matches in a string instead of stopping at the first one?

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