validate the selector

packages/tailwindcss/src/index.ts

@@ -4,6 +4,7 @@ import { compileCandidates } from './compile'
 import * as CSS from './css-parser'
 import { buildDesignSystem } from './design-system'
 import { Theme } from './theme'
+import { isSimpleClassSelector } from './utils/is-simple-class-selector'
 
 export function compile(css: string): {
   build(candidates: string[]): string
@@ -33,7 +34,15 @@ export function compile(css: string): {
     if (node.kind !== 'rule') return
 
     // Track all user-defined classes for `@apply` support
-    if (containsAtApply && node.selector[0] === '.' && !node.selector.includes(' ')) {
+    if (
+      containsAtApply &&
+      // Verify that it is a valid applyable-class. An applyable class is a
+      // class that is a very simple selector, like `.foo` or `.bar`, but doesn't
+      // contain any spaces, combinators, pseudo-selectors, pseudo-elements, or
+      // attribute selectors.
+      node.selector[0] === '.' &&
+      isSimpleClassSelector(node.selector)
+    ) {
       // Convert the class `.foo` into a candidate `foo`
       let candidate = node.selector.slice(1)
 

packages/tailwindcss/src/utils/is-simple-class-selector.test.ts

@@ -0,0 +1,39 @@
+import { expect, it } from 'vitest'
+import { isSimpleClassSelector } from './is-simple-class-selector'
+
+it.each([
+  // Simple class selector
+  ['.foo', true],
+
+  // Class selectors with escaped characters
+  ['.w-\\[123px\\]', true],
+  ['.content-\\[\\+\\>\\~\\*\\]', true],
+
+  // ID selector
+  ['#foo', false],
+  ['.foo#foo', false],
+
+  // Element selector
+  ['h1', false],
+  ['h1.foo', false],
+
+  // Attribute selector
+  ['[data-foo]', false],
+  ['.foo[data-foo]', false],
+  ['[data-foo].foo', false],
+
+  // Pseudo-class selector
+  ['.foo:hover', false],
+
+  // Combinator
+  ['.foo>.bar', false],
+  ['.foo+.bar', false],
+  ['.foo~.bar', false],
+  ['.foo .bar', false],
+
+  // Selector list
+  ['.foo, .bar', false],
+  ['.foo,.bar', false],
+])('should validate %s', (selector, expected) => {
+  expect(isSimpleClassSelector(selector)).toBe(expected)
+})

packages/tailwindcss/src/utils/is-simple-class-selector.ts

@@ -0,0 +1,25 @@
+export function isSimpleClassSelector(selector: string): boolean {
+  // The selector must start with a dot, otherwise it's not a class selector.
+  if (selector[0] !== '.') return false
+
+  for (let i = 1; i < selector.length; i++) {
+    switch (selector[i]) {
+      // The character is escaped, skip the next character
+      case '\\':
+        i += 1
+        continue
+
+      case ' ': // Descendat combinator
+      case '#': // ID selector
+      case '[': // Attribute selector
+      case ':': // Pseudo-classes and pseudo-elements
+      case '>': // Child combinator
+      case '+': // Next-sibling combinator
+      case '~': // Subsequent-sibling combinator
+      case ',': // Selector list
+        return false
+    }
+  }
+
+  return true
+}