diff --git a/JSTests/stress/regexp-ignore-case-unicode-16-17-case-pairs.js b/JSTests/stress/regexp-ignore-case-unicode-16-17-case-pairs.js
new file mode 100644
index 000000000000..9ae738da2ab9
--- /dev/null
+++ b/JSTests/stress/regexp-ignore-case-unicode-16-17-case-pairs.js
@@ -0,0 +1,96 @@
+//@ runDefault("--useRegExpJIT=true")
+//@ runNoJIT("--useRegExpJIT=false")
+
+// Non-unicode /i matching uses the committed table in YarrCanonicalizeUCS2.cpp.
+// That table has to contain the BMP simple case pairs added in Unicode 16 and 17;
+// /iu (generated from ucd/CaseFolding.txt at build time) already had them.
+
+function shouldBe(actual, expected, message) {
+    if (actual !== expected)
+        throw new Error(message + ": expected " + expected + " but got " + actual);
+}
+
+function escapeForClass(ch) {
+    return "\\u" + ch.charCodeAt(0).toString(16).padStart(4, "0");
+}
+
+function checkPair(lower, upper, flags) {
+    const lo = String.fromCharCode(lower);
+    const up = String.fromCharCode(upper);
+    const name = "U+" + lower.toString(16) + "/U+" + upper.toString(16) + " /" + flags;
+
+    for (const [pattern, input] of [[lo, up], [up, lo]]) {
+        const escaped = escapeForClass(pattern);
+        shouldBe(new RegExp(escaped, flags).test(input), true, name + " atom");
+        shouldBe(new RegExp("^" + escaped + "$", flags).test(input), true, name + " anchored atom");
+        shouldBe(new RegExp("[" + escaped + "]", flags).test(input), true, name + " class");
+        shouldBe(new RegExp("[^" + escaped + "]", flags).test(input), false, name + " negated class");
+        shouldBe(new RegExp("x" + escaped + "y", flags).test("x" + input + "y"), true, name + " inside a longer atom");
+        shouldBe(new RegExp("(" + escaped + ")\\1", flags).test(pattern + input), true, name + " backreference");
+        shouldBe(("a" + input + "b").replace(new RegExp(escaped, flags), "-"), "a-b", name + " replace");
+    }
+}
+
+function checkUnrelated(a, b, flags) {
+    const name = "U+" + a.toString(16) + " vs U+" + b.toString(16) + " /" + flags;
+    shouldBe(new RegExp(escapeForClass(String.fromCharCode(a)), flags).test(String.fromCharCode(b)), false, name + " atom");
+    shouldBe(new RegExp("[" + escapeForClass(String.fromCharCode(a)) + "]", flags).test(String.fromCharCode(b)), false, name + " class");
+}
+
+// [lower, upper]
+const newPairs = [
+    // Unicode 16
+    [0x019b, 0xa7dc], // LATIN LETTER LAMBDA WITH STROKE
+    [0x0264, 0xa7cb], // LATIN LETTER RAMS HORN
+    [0xa7cd, 0xa7cc], // LATIN LETTER S WITH DIAGONAL STROKE
+    [0xa7db, 0xa7da], // LATIN LETTER LAMBDA
+    [0x1c8a, 0x1c89], // CYRILLIC LETTER TJE
+    // Unicode 17
+    [0xa7cf, 0xa7ce], // LATIN LETTER PHARYNGEAL VOICED FRICATIVE
+    [0xa7d3, 0xa7d2], // LATIN LETTER DOUBLE THORN (capital is new, the small letter is from Unicode 14)
+    [0xa7d5, 0xa7d4], // LATIN LETTER DOUBLE WYNN (same)
+];
+
+// Pairs next to the new entries whose table ranges were merged or split by the update.
+const neighbouringPairs = [
+    [0x019a, 0x023d], // LATIN LETTER L WITH BAR
+    [0x0263, 0x0194], // LATIN LETTER GAMMA
+    [0x0265, 0xa78d], // LATIN LETTER TURNED H
+    [0xa7d1, 0xa7d0], // LATIN LETTER CLOSED INSULAR G
+    [0xa7d7, 0xa7d6], // LATIN LETTER MIDDLE SCOTS S
+    [0xa7d9, 0xa7d8], // LATIN LETTER SIGMOID S
+    [0xa7f6, 0xa7f5], // LATIN LETTER REVERSED HALF H
+    [0xa64b, 0xa64a], // CYRILLIC LETTER MONOGRAPH UK (also equivalent to U+1C88 below)
+];
+
+for (let i = 0; i < 50; ++i) {
+    for (const [lower, upper] of newPairs) {
+        checkPair(lower, upper, "i");
+        checkPair(lower, upper, "iu");
+    }
+    for (const [lower, upper] of neighbouringPairs) {
+        checkPair(lower, upper, "i");
+        checkPair(lower, upper, "iu");
+    }
+
+    // U+1C88 CYRILLIC SMALL LETTER UNBLENDED UK is in the same set as U+A64A/U+A64B.
+    checkPair(0x1c88, 0xa64a, "i");
+    checkPair(0x1c88, 0xa64b, "i");
+
+    // The new pairs must not leak into the code units around them.
+    checkUnrelated(0x1c8a, 0x1c8b, "i"); // U+1C8B is unassigned
+    checkUnrelated(0x1c89, 0x1c88, "i");
+    checkUnrelated(0xa7cb, 0xa7ca, "i"); // U+A7CA pairs with U+A7C9
+    checkUnrelated(0xa7dc, 0xa7dd, "i"); // U+A7DD is unassigned
+    checkUnrelated(0xa7dc, 0xa7db, "i");
+    checkUnrelated(0x019b, 0x019a, "i");
+    checkUnrelated(0x0264, 0x0263, "i");
+}
+
+// Case-insensitive ranges in classes cover the new partners too. Each tested
+// character lies outside the range and can only match through its partner.
+shouldBe(/[\ua7cc-\ua7da]/i.test("\ua7db"), true, "U+A7DB matches through U+A7DA at the end of the range");
+shouldBe(/[\u0190-\u01a0]/i.test("\ua7dc"), true, "U+A7DC matches through U+019B inside the range");
+shouldBe(/[\ua7c0-\ua7ff]/i.test("\u019b"), true, "U+019B matches through U+A7DC inside the range");
+shouldBe(/[\ua7c0-\ua7ff]/i.test("\u0264"), true, "U+0264 matches through U+A7CB inside the range");
+shouldBe(/[\u1c80-\u1c89]/i.test("\u1c8a"), true, "U+1C8A matches through U+1C89 at the end of the range");
diff --git a/Source/JavaScriptCore/yarr/YarrCanonicalizeUCS2.cpp b/Source/JavaScriptCore/yarr/YarrCanonicalizeUCS2.cpp
index 010a55370de5..6cc72dba5244 100644
--- a/Source/JavaScriptCore/yarr/YarrCanonicalizeUCS2.cpp
+++ b/Source/JavaScriptCore/yarr/YarrCanonicalizeUCS2.cpp
@@ -128,7 +128,7 @@ constinit const CanonicalizationRange ucs2RangeInfo[ucs2CanonicalizationRanges]
     { 0x0197, 0x0197, 0x00d1, CanonicalizeRangeLo },
     { 0x0198, 0x0199, 0x0000, CanonicalizeAlternatingAligned },
     { 0x019a, 0x019a, 0x00a3, CanonicalizeRangeLo },
-    { 0x019b, 0x019b, 0x0000, CanonicalizeUnique },
+    { 0x019b, 0x019b, 0xa641, CanonicalizeRangeLo },
     { 0x019c, 0x019c, 0x00d3, CanonicalizeRangeLo },
     { 0x019d, 0x019d, 0x00d5, CanonicalizeRangeLo },
     { 0x019e, 0x019e, 0x0082, CanonicalizeRangeLo },
@@ -193,7 +193,7 @@ constinit const CanonicalizationRange ucs2RangeInfo[ucs2CanonicalizationRanges]
     { 0x0261, 0x0261, 0xa54b, CanonicalizeRangeLo },
     { 0x0262, 0x0262, 0x0000, CanonicalizeUnique },
     { 0x0263, 0x0263, 0x00cf, CanonicalizeRangeHi },
-    { 0x0264, 0x0264, 0x0000, CanonicalizeUnique },
+    { 0x0264, 0x0264, 0xa567, CanonicalizeRangeLo },
     { 0x0265, 0x0265, 0xa528, CanonicalizeRangeLo },
     { 0x0266, 0x0266, 0xa544, CanonicalizeRangeLo },
     { 0x0267, 0x0267, 0x0000, CanonicalizeUnique },
@@ -369,7 +369,8 @@ constinit const CanonicalizationRange ucs2RangeInfo[ucs2CanonicalizationRanges]
     { 0x1c86, 0x1c86, 0x0013, CanonicalizeSet },
     { 0x1c87, 0x1c87, 0x0014, CanonicalizeSet },
     { 0x1c88, 0x1c88, 0x0016, CanonicalizeSet },
-    { 0x1c89, 0x1c8f, 0x0000, CanonicalizeUnique },
+    { 0x1c89, 0x1c8a, 0x0000, CanonicalizeAlternatingUnaligned },
+    { 0x1c8b, 0x1c8f, 0x0000, CanonicalizeUnique },
     { 0x1c90, 0x1cba, 0x0bc0, CanonicalizeRangeHi },
     { 0x1cbb, 0x1cbc, 0x0000, CanonicalizeUnique },
     { 0x1cbd, 0x1cbf, 0x0bc0, CanonicalizeRangeHi },
@@ -529,11 +530,10 @@ constinit const CanonicalizationRange ucs2RangeInfo[ucs2CanonicalizationRanges]
     { 0xa7c5, 0xa7c5, 0xa543, CanonicalizeRangeHi },
     { 0xa7c6, 0xa7c6, 0x8a38, CanonicalizeRangeHi },
     { 0xa7c7, 0xa7ca, 0x0000, CanonicalizeAlternatingUnaligned },
-    { 0xa7cb, 0xa7cf, 0x0000, CanonicalizeUnique },
-    { 0xa7d0, 0xa7d1, 0x0000, CanonicalizeAlternatingAligned },
-    { 0xa7d2, 0xa7d5, 0x0000, CanonicalizeUnique },
-    { 0xa7d6, 0xa7d9, 0x0000, CanonicalizeAlternatingAligned },
-    { 0xa7da, 0xa7f4, 0x0000, CanonicalizeUnique },
+    { 0xa7cb, 0xa7cb, 0xa567, CanonicalizeRangeHi },
+    { 0xa7cc, 0xa7db, 0x0000, CanonicalizeAlternatingAligned },
+    { 0xa7dc, 0xa7dc, 0xa641, CanonicalizeRangeHi },
+    { 0xa7dd, 0xa7f4, 0x0000, CanonicalizeUnique },
     { 0xa7f5, 0xa7f6, 0x0000, CanonicalizeAlternatingUnaligned },
     { 0xa7f7, 0xab52, 0x0000, CanonicalizeUnique },
     { 0xab53, 0xab53, 0x03a0, CanonicalizeRangeHi },