\ are valid arbitrary variant tokens

We use `\` for escaping `.` or `_` so they should be part of the
arbitrary variant regex.

Author: RobinMalfait

src/lib/defaultExtractor.js

@@ -69,7 +69,7 @@ function* buildRegExps(context) {
     '((?=((',
     regex.any(
       [
-        regex.pattern([/([^\s"'`\[\\]+-)?\[[^\s"'`\\]+\]/, separator]),
+        regex.pattern([/([^\s"'`\[\\]+-)?\[[^\s"'`]+\]/, separator]),
         regex.pattern([/[^\s"'`\[\\]+/, separator]),
       ],
       true

tests/arbitrary-variants.test.js

@@ -405,3 +405,57 @@ test('with @apply', () => {
     `)
   })
 })
+
+test('keeps escaped underscores', () => {
+  let config = {
+    content: [
+      {
+        raw: '<div class="[&_.foo\\_\\_bar]:underline"></div>',
+      },
+    ],
+    corePlugins: { preflight: false },
+  }
+
+  let input = `
+    @tailwind base;
+    @tailwind components;
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      ${defaults}
+
+      .\[\&_\.foo\\_\\_bar\]\:underline .foo__bar {
+        text-decoration-line: underline;
+      }
+    `)
+  })
+})
+
+test('keeps escaped underscores with multiple arbitrary variants', () => {
+  let config = {
+    content: [
+      {
+        raw: '<div class="[&_.foo\\_\\_bar]:[&_.bar\\_\\_baz]:underline"></div>',
+      },
+    ],
+    corePlugins: { preflight: false },
+  }
+
+  let input = `
+    @tailwind base;
+    @tailwind components;
+    @tailwind utilities;
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      ${defaults}
+
+      .\[\&_\.foo\\_\\_bar\]\:\[\&_\.bar\\_\\_baz\]\:underline .bar__baz .foo__bar {
+        text-decoration-line: underline;
+      }
+    `)
+  })
+})