Do not allow variants to end with - or _ (#18872)

This PR is a followup of #18867, but this time we won't allow
`@custom-variant` to end with `-` or `_`.

The same reasoning applies here where Oxide doesn't pick this up but
Intellisense and Tailwind CSS' core does.

---------

Co-authored-by: Jordan Pittman <thecrypticace@gmail.com>

CHANGELOG.md

@@ -22,7 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Hide internal fields from completions in `matchUtilities` ([#18820](https://github.com/tailwindlabs/tailwindcss/pull/18820))
 - Ignore `.vercel` folders by default (can be overridden by `@source …` rules) ([#18855](https://github.com/tailwindlabs/tailwindcss/pull/18855))
 - Consider variants starting with `@-` to be invalid (e.g. `@-2xl:flex`) ([#18869](https://github.com/tailwindlabs/tailwindcss/pull/18869))
-- Do not allow custom variants to start with a `-` ([#18867](https://github.com/tailwindlabs/tailwindcss/pull/18867))
+- Do not allow custom variants to start or end with a `-` or `_` ([#18867](https://github.com/tailwindlabs/tailwindcss/pull/18867), [#18872](https://github.com/tailwindlabs/tailwindcss/pull/18872))
 - Upgrade: Migrate `aria` theme keys to `@custom-variant` ([#18815](https://github.com/tailwindlabs/tailwindcss/pull/18815))
 - Upgrade: Migrate `data` theme keys to `@custom-variant` ([#18816](https://github.com/tailwindlabs/tailwindcss/pull/18816))
 - Upgrade: Migrate `supports` theme keys to `@custom-variant` ([#18817](https://github.com/tailwindlabs/tailwindcss/pull/18817))

packages/tailwindcss/src/index.test.ts

@@ -3772,24 +3772,31 @@ describe('@custom-variant', () => {
     )
   })
 
-  test('@custom-variant cannot contain dashes on its own', () => {
-    return expect(
-      compileCss(css`
-        @custom-variant - (&.dash);
-      `),
-    ).rejects.toThrowErrorMatchingInlineSnapshot(
-      `[Error: \`@custom-variant -\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.]`,
-    )
-  })
+  test.each([
+    // Cannot be a dash on its own
+    [`@custom-variant - (&);`],
+    // Cannot be multiple dashes on their own
+    [`@custom-variant --- (&);`],
+    // Cannot be an underscore on its own
+    [`@custom-variant _ (&);`],
+    // Cannot be multiple underscores on their own
+    [`@custom-variant ___ (&);`],
 
-  test('@custom-variant cannot contain multiple dashes on their own', () => {
-    return expect(
-      compileCss(css`
-        @custom-variant --- (&.dashed);
-      `),
-    ).rejects.toThrowErrorMatchingInlineSnapshot(
-      `[Error: \`@custom-variant ---\` defines an invalid variant name. Variants should only contain alphanumeric, dashes, or underscore characters and start with a lowercase letter or number.]`,
-    )
+    // Cannot start with a dash
+    [`@custom-variant -foo (&);`],
+    [`@custom-variant --foo (&);`],
+    // Cannot start with an underscore
+    [`@custom-variant _foo (&);`],
+    [`@custom-variant __foo (&);`],
+
+    // Cannot end with a dash
+    [`@custom-variant foo- (&);`],
+    [`@custom-variant foo-- (&);`],
+    // Cannot end with an underscore
+    [`@custom-variant foo_ (&);`],
+    [`@custom-variant foo__ (&);`],
+  ])('@custom-variant must have a valid name', (input) => {
+    return expect(compileCss(input)).rejects.toThrowError()
   })
 
   test('@custom-variant must not container special characters', () => {

packages/tailwindcss/src/variants.ts

@@ -18,7 +18,7 @@ import { DefaultMap } from './utils/default-map'
 import { isPositiveInteger } from './utils/infer-data-type'
 import { segment } from './utils/segment'
 
-export const IS_VALID_VARIANT_NAME = /^@?[a-z0-9][a-zA-Z0-9_-]*$/
+export const IS_VALID_VARIANT_NAME = /^@?[a-z0-9][a-zA-Z0-9_-]*(?<![_-])$/
 
 type VariantFn<T extends Variant['kind']> = (
   rule: Rule,