add a test for \w regex under /iu flags

test/built-ins/RegExp/unicode_character_class_word_escape.js

@@ -0,0 +1,21 @@
+// Copyright (C) 2026 Nikita Skovoroda. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-wordcharacters
+description: >
+  \w with case-insensitive flag behaves differently in Unicode patterns.
+info: |
+  WordCharacters ( rer )
+  The abstract operation WordCharacters takes argument rer (a RegExp Record) and returns a CharSet. Returns a CharSet containing the characters considered "word characters" for the purposes of \b, \B, \w, and \W It performs the following steps when called:
+
+  1. Let basicWordChars be the CharSet containing every character in the ASCII word characters.
+  2. Let extraWordChars be the CharSet containing all characters c such that c is not in basicWordChars but Canonicalize(rer, c) is in basicWordChars.
+  3. Assert: extraWordChars is empty unless HasEitherUnicodeFlag(rer) is true and rer.[[IgnoreCase]] is true.
+  4. Return the union of basicWordChars and extraWordChars.
+---*/
+
+assert.sameValue(/\w/.test('\u212a'), false);
+assert.sameValue(/\w/u.test('\u212a'), false);
+assert.sameValue(/\w/i.test('\u212a'), false);
+assert.sameValue(/\w/ui.test('\u212a'), true);