RegExp.prototype.ignoreCase
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
TheignoreCase accessor property ofRegExp instances returns whether or not thei flag is used with this regular expression.
Try it
const regex1 = /foo/;const regex2 = /foo/i;console.log(regex1.test("Football"));// Expected output: falseconsole.log(regex2.ignoreCase);// Expected output: trueconsole.log(regex2.test("Football"));// Expected output: trueDescription
RegExp.prototype.ignoreCase has the valuetrue if thei flag was used; otherwise,false. Thei flag indicates that case should be ignored while attempting a match in a string. Case-insensitive matching is done by mapping both the expected character set and the matched string to the same casing.
If the regex isUnicode-aware, the case mapping happens throughsimple case folding specified inCaseFolding.txt. The mapping always maps to a single code point, so it does not map, for example,ß (U+00DF LATIN SMALL LETTER SHARP S) toss (which isfull case folding, notsimple case folding). It may however map code points outside the Basic Latin block to code points within it — for example,ſ (U+017F LATIN SMALL LETTER LONG S) case-folds tos (U+0073 LATIN SMALL LETTER S) andK (U+212A KELVIN SIGN) case-folds tok (U+006B LATIN SMALL LETTER K). Therefore,ſ andK can be matched by/[a-z]/ui.
If the regex is Unicode-unaware, case mapping uses theUnicode Default Case Conversion — the same algorithm used inString.prototype.toUpperCase(). This algorithm prevents code points outside the Basic Latin block to be mapped to code points within it, soſ andK mentioned previously are not matched by/[a-z]/i.
Unicode-aware case folding generally folds to lower case, while Unicode-unaware case folding folds to upper case. These two are not perfect reverse operations, so there are some subtle behavior differences. For example,Ω (U+2126 OHM SIGN) andΩ (U+03A9 GREEK CAPITAL LETTER OMEGA) are both mapped by simple case folding toω (U+03C9 GREEK SMALL LETTER OMEGA), so"\u2126" is matched by/[\u03c9]/ui and/[\u03a9]/ui; on the other hand, U+2126 is mapped by Default Case Conversion to itself, while the other two both map to U+03A9, so"\u2126" is matched by neither/[\u03c9]/i nor/[\u03a9]/i.
The set accessor ofignoreCase isundefined. You cannot change this property directly.
Examples
Using ignoreCase
const regex = /foo/i;console.log(regex.ignoreCase); // trueSpecifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-get-regexp.prototype.ignorecase |