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JavaScript RegExp i Modifier - GeeksforGeeks

JavaScript RegExp i Modifier

Last Updated : 11 Jul, 2025

The i modifier in JavaScript regular expressions stands for case-insensitivity. It allows the regex to match letters in a string regardless of their case, making it ideal for scenarios where matching should not be case-sensitive, such as user input validation or text search.

When the i flag is active, both uppercase and lowercase forms of letters are treated as equivalent.

JavaScript


 // Regular expression without 'i' flag
let regex1 = /hello/;
console.log(regex1.test("Hello"));

// Regular expression with 'i' flag
let regex2 = /hello/i;
console.log(regex2.test("Hello"));

Without i flag: The regex is case-sensitive and only matches "hello" in lowercase.
With i flag: The regex becomes case-insensitive, matching both "hello" and "Hello."

Syntax

let regex = /pattern/i;

Key Points

Case-Insensitive Matching: Matches both uppercase and lowercase characters.
Readability: Simplifies patterns by removing the need for explicitly defining cases (e.g., [Hh]).
Scope: Applies to all characters in the regex, not just specific sections.

Real-World Examples of the i Modifier 1. Simple Word Match JavaScript


 let regex = /javascript/i;

console.log(regex.test("JavaScript"));
console.log(regex.test("JAVASCRIPT"));
console.log(regex.test("javaScript"));

2. Matching Case-Insensitive Substrings JavaScript


 let regex = /error/i;

console.log("Error found!".match(regex));
console.log("no ERROR here".match(regex));

Output

[ 'Error', index: 0, input: 'Error found!', groups: undefined ]
[ 'ERROR', index: 3, input: 'no ERROR here', groups: undefined ]

3. Validating Case-Insensitive Email Domains JavaScript


 let email = "user@Gmail.com";
let regex = /@gmail\.com/i;

console.log(regex.test(email));

4. Checking for Case-Insensitive Keywords JavaScript


 let s = "This is a simple example.";
let regex = /SIMPLE/i;

console.log(regex.test(s));

Why Use the i Modifier?

Case Flexibility: Simplifies matching in cases where case does not matter.
Cleaner Regex: Avoids the need for alternation patterns like [A-Za-z].
User Input Handling: Ensures consistent matching for input validation.

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