fix: match bare basePath root for catch-all middleware matchers

packages/next/src/build/analysis/get-page-static-info.test.ts

@@ -86,5 +86,51 @@ describe('get-page-static-infos', () => {
         regex.test('/index.segments/$c$children/__PAGE__.segment.rsc')
       ).toBe(true)
     })
+
+    it('matches the bare basePath root for a catch-all matcher', () => {
+      // Regression test for https://github.com/vercel/next.js/issues/73786
+      const matcher = '/((?!api|_next/static|_next/image|favicon.ico).*)'
+      const regex = new RegExp(
+        getMiddlewareMatchers(matcher, {
+          i18n: undefined,
+          basePath: '/docs',
+        })[0].regexp
+      )
+
+      // The bare basePath root must match (previously it did not).
+      expect(regex.test('/docs')).toBe(true)
+      expect(regex.test('/docs/')).toBe(true)
+      expect(regex.test('/docs/foo')).toBe(true)
+      // Negative lookahead and word boundaries must still hold.
+      expect(regex.test('/docs/api/x')).toBe(false)
+      expect(regex.test('/docsfoo')).toBe(false)
+      expect(regex.test('/foo')).toBe(false)
+    })
+
+    it('does not match the bare basePath root for a specific matcher', () => {
+      const regex = new RegExp(
+        getMiddlewareMatchers('/dashboard', {
+          i18n: undefined,
+          basePath: '/docs',
+        })[0].regexp
+      )
+
+      expect(regex.test('/docs/dashboard')).toBe(true)
+      // A non-root matcher should not be widened to match the basePath root.
+      expect(regex.test('/docs')).toBe(false)
+      expect(regex.test('/docs/')).toBe(false)
+    })
+
+    it('does not match the bare basePath root for a param matcher', () => {
+      const regex = new RegExp(
+        getMiddlewareMatchers('/:id', {
+          i18n: undefined,
+          basePath: '/docs',
+        })[0].regexp
+      )
+
+      expect(regex.test('/docs/apple')).toBe(true)
+      expect(regex.test('/docs')).toBe(false)
+    })
   })
 })

packages/next/src/build/analysis/get-page-static-info.ts

@@ -492,6 +492,19 @@ export function getMiddlewareMatchers(
     }`
     source = `${OPTIONAL_MIDDLEWARE_NEXT_DATA_PREFIX}${source}${sourceSuffix}`
 
+    // Whether this matcher (before prepending `basePath`) matches the site
+    // root `/`. A catch-all like `/((?!api).*)` matches `/` because its single
+    // optional capture group can be empty. We need to know this before adding
+    // `basePath` so we can keep the bare `basePath` root matching too (see
+    // below).
+    const matchesRoot =
+      !!nextConfig.basePath &&
+      !isRoot &&
+      (() => {
+        const { regexStr } = tryToParsePath(source)
+        return regexStr ? new RegExp(regexStr).test('/') : false
+      })()
+
     if (nextConfig.basePath) {
       source = `${nextConfig.basePath}${source}`
     }
@@ -507,11 +520,23 @@ export function getMiddlewareMatchers(
       process.exit(1)
     }
 
+    // We know that parsed.regexStr is not undefined because we already
+    // checked that the source is valid.
+    let regexp = tryToParsePath(result.data).regexStr!
+
+    // When `basePath` is set, prepending it turns a matcher that previously
+    // matched `/` into one anchored at `${basePath}/...`, so the bare
+    // `basePath` root (e.g. `/docs`) no longer matches even though `/` did
+    // without a `basePath`. Restore that by also accepting the bare
+    // `basePath` root. See https://github.com/vercel/next.js/issues/73786.
+    if (matchesRoot) {
+      const basePathRoot = `${escapeStringRegexp(nextConfig.basePath!)}[\\/#\\?]?$`
+      regexp = `^(?:${basePathRoot}|${regexp.replace(/^\^/, '')})`
+    }
+
     return {
       ...rest,
-      // We know that parsed.regexStr is not undefined because we already
-      // checked that the source is valid.
-      regexp: tryToParsePath(result.data).regexStr!,
+      regexp,
       originalSource: originalSource || source,
     }
   })