JavaScript RegExp ignoreCase Property

What Is ignoreCase?

ignoreCase is a read-only boolean on a RegExp object. It is true when the pattern was created with the i flag — so letter matching ignores uppercase vs lowercase.

/hello/.test("HELLO")   → false  (case-sensitive)
/hello/i.test("HELLO")  → true   (ignoreCase)

/hello/.ignoreCase      → false
/hello/i.ignoreCase     → true

new RegExp("ok", "i").ignoreCase  → true

// i affects letters — not digits or symbols

How to Use ignoreCase

Add the i flag when you create the regex, then read .ignoreCase if you need to inspect it.

1. Write /pattern/i (or new RegExp("pattern", "i")).
2. Use test() or match() — letters match either case.
3. Read regex.ignoreCase to confirm the flag is set.
4. Combine with g as /pattern/gi when you need every match.

Pseudocode

if regex has flag i:
    ignoreCase = true
    when matching letters:
        treat "A" and "a" as the same
else:
    ignoreCase = false
    "A" and "a" are different

// digits and most symbols stay unchanged either way

Cheat Sheet

Keep these facts handy while writing case-insensitive patterns.

Live Preview

Compare /error/ vs /error/i on a sample string.

text = "ERROR found"
/error/.test(text)   → false
/error/i.test(text)  → true

/error/.ignoreCase   → false
/error/i.ignoreCase  → true

JavaScript Examples

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

const text = "Hello World";

console.log(/hello/.test(text));   // false
console.log(/hello/i.test(text));  // true
console.log(/WORLD/i.test(text));  // true

false
true
true

const text = "Error ERROR error";

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

error
Error,ERROR,error
null

const a = /ok/;
const b = /ok/i;
const c = new RegExp("ok", "i");

console.log(a.ignoreCase);  // false
console.log(b.ignoreCase);  // true
console.log(c.ignoreCase);  // true
console.log(b.flags);       // "i"

false
true
true
i

Performance Notes

The i flag adds negligible cost for typical patterns. Prefer test when you only need a yes/no answer.

Key Takeaways