JavaScript RegExp i Flag

What Is the i Flag?

The i flag (ignoreCase) makes letter matching case-insensitive. With /hello/i, the pattern matches hello, Hello, and HELLO—ideal for user input and search where case should not matter.

/pattern/i           // ignore letter case
regex.ignoreCase     // true when i is set

/hello/i.test("HELLO")  → true
/hello/.test("HELLO")   → false

digits & symbols: unchanged by i
combine: /error/gi

How to Use the i Flag

Add i, then test, match, or replace as usual.

1. Create a case-insensitive regex: /pattern/i or new RegExp("pattern", "i").
2. Call test, match, or replace — letters fold case automatically.
3. Add g when you need every occurrence: /pattern/gi.
4. Confirm with regex.ignoreCase when debugging.

Pseudocode

re = /pattern/i

for each letter in pattern and input:
    compare using case fold
    (A ≡ a, B ≡ b, …)

digits / symbols:
    compare exactly (i has no effect)

if all parts match → success

Cheat Sheet

Keep these ignoreCase facts handy.

Live Preview

Compare case-sensitive vs case-insensitive matching on sample text.

/hello/i.test("Hello")
→ true

/* i folds letter case */

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

console.log(/hello/i.test("Hello World"));  // true
console.log(/hello/i.test("HELLO"));        // true
console.log(/hello/.test("Hello World"));   // false

console.log(/hello/i.ignoreCase);           // true
console.log(/hello/.ignoreCase);            // false

true
true
false
true
false

const text = "Error: file not found. ERROR logged.";

console.log(text.match(/error/gi));  // ["Error", "ERROR"]
console.log(text.match(/error/g));   // null (exact "error" missing)

[ 'Error', 'ERROR' ]
null

const text = "I love js and JS and Js";

console.log(text.replace(/js/gi, "JavaScript"));
// "I love JavaScript and JavaScript and JavaScript"

console.log(text.replace(/js/g, "JavaScript"));
// "I love JavaScript and JS and Js"  (only lowercase js)

I love JavaScript and JavaScript and JavaScript
I love JavaScript and JS and Js

Performance Notes

Case folding is handled by the engine. Prefer /pattern/i over building large character classes or lowercasing the whole string first unless you have a specific reason.

Key Takeaways