feat: handle backslash in @utility name (#19626)

Resolves #19607

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## Summary

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Provide a summary of the issue and the changes you're making. How does
your change solve the problem?

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I believe there is no obstacle to simply ignoring backslashes in the
name. This way, various validators - which are not aware of Tailwind
CSS's specific syntax (which allows the `/` character to be used
directly in utility names) - can still be bypassed using backslashes.
For example, instead of `@utility push-1/2`, one could use `@utility
push-1\/2`, while the end result would be identical.

## Test plan

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I took a previous utility test as a baseline and extended it with
backslashes, and I expect the same result in the output as in the
original test case:

*
https://github.com/rozsazoltan/tailwindcss/blob/main/packages/tailwindcss/src/utilities.test.ts#L28553-L28567
(the original test case I started from)
*
https://github.com/rozsazoltan/tailwindcss/blob/feat/handle-blackslash-in-utility-name/packages/tailwindcss/src/index.test.ts#L4640-L4654
(the current PR's test)

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure changes to external files listed via `@source` trigger a full page reload when using `@tailwindcss/vite` ([#19670](https://github.com/tailwindlabs/tailwindcss/pull/19670))
 - Improve performance Oxide scanner in bigger projects ([#19632](https://github.com/tailwindlabs/tailwindcss/pull/19632))
 - Ensure import aliases in Astro v5 work without crashing ([#19677](https://github.com/tailwindlabs/tailwindcss/issues/19677))
+- Allow escape characters in `@utility` names to improve support with formatters such as Biome ([#19626](https://github.com/tailwindlabs/tailwindcss/pull/19626))
 
 ### Deprecated
 

packages/tailwindcss/src/index.test.ts

@@ -4729,6 +4729,31 @@ describe('@utility', () => {
     )
   })
 
+  test('@utility can handle escape sequences correctly', async () => {
+    let { build } = await compile(css`
+      @layer utilities {
+        @tailwind utilities;
+      }
+
+      @utility push-1\/2 {
+        right: 50%;
+      }
+
+      @utility push-50\% {
+        right: 50%;
+      }
+    `)
+    let compiled = build(['push-1/2', 'push-50%'])
+
+    expect(optimizeCss(compiled).trim()).toMatchInlineSnapshot(`
+      "@layer utilities {
+        .push-1\\/2, .push-50\\% {
+          right: 50%;
+        }
+      }"
+    `)
+  })
+
   test('A functional @utility must end in -*', () => {
     return expect(
       compileCss(css`

packages/tailwindcss/src/utilities.ts

@@ -16,6 +16,7 @@ import { enableContainerSizeUtility } from './feature-flags'
 import type { Theme, ThemeKey } from './theme'
 import { compareBreakpoints } from './utils/compare-breakpoints'
 import { DefaultMap } from './utils/default-map'
+import { unescape } from './utils/escape'
 import {
   inferDataType,
   isPositiveInteger,
@@ -5939,7 +5940,11 @@ export const BARE_VALUE_DATA_TYPES = [
 ]
 
 export function createCssUtility(node: AtRule) {
-  let name = node.params
+  // Allow escaped characters in the name for compatibility with formatters and
+  // other parsers, to ensure valid CSS syntax. E.g.: `@utility foo-1\/2`.
+  //
+  // Note: the actual utility will be `foo-1/2`
+  let name = unescape(node.params)
 
   // Functional utilities. E.g.: `tab-size-*`
   if (isValidFunctionalUtilityName(name)) {