diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3641da5e1..554bad8dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Ensure classes are extracted when a variant precedes an arbitrary variant containing a quoted attribute selector (e.g. `focus-visible:[&:not([aria-selected="true"])]:bg-red-500`) ([#20374](https://github.com/tailwindlabs/tailwindcss/pull/20374))
 
 ## [3.4.19] - 2025-12-10
 
diff --git a/src/lib/defaultExtractor.js b/src/lib/defaultExtractor.js
index 34407794f..f9e9783e2 100644
--- a/src/lib/defaultExtractor.js
+++ b/src/lib/defaultExtractor.js
@@ -133,7 +133,7 @@ function* buildRegExps(context) {
       regex.pattern([/([^\s"'`\[\\]+-)?\[[^\s`]+\]\/[\w_-]+/, separator]),
 
       regex.pattern([/([^\s"'`\[\\]+-)?\[[^\s`]+\]/, separator]),
-      regex.pattern([/[^\s`\[\\]+/, separator]),
+      regex.pattern([/[^\s"'`\[\\]+/, separator]),
     ]),
   ]
 
diff --git a/tests/default-extractor.test.js b/tests/default-extractor.test.js
index a0b6384ef..9d319bbc7 100644
--- a/tests/default-extractor.test.js
+++ b/tests/default-extractor.test.js
@@ -576,3 +576,14 @@ it.each([
     expect(extractions).toContain(value)
   }
 })
+
+// https://github.com/tailwindlabs/tailwindcss/issues/18387
+test('arbitrary not selector with preceding variants', () => {
+  let extractions = defaultExtractor(`
+    <div class="focus-visible:[&:not([aria-selected='true'])]:bg-red-500"></div>
+    <div class='hover:[&:not([aria-selected="true"])]:bg-red-500'></div>
+  `)
+
+  expect(extractions).toContain("focus-visible:[&:not([aria-selected='true'])]:bg-red-500")
+  expect(extractions).toContain('hover:[&:not([aria-selected="true"])]:bg-red-500')
+})