diff --git a/src/path.ts b/src/path.ts
index f613336..48f5e1e 100644
--- a/src/path.ts
+++ b/src/path.ts
@@ -182,10 +182,10 @@ export const toNamespacedPath: typeof path.toNamespacedPath = function (p) {
 }
 
 // extname
-const _EXTNAME_RE = /(?<!^)\.[^/.]+$/
+const _EXTNAME_RE = /.(\.[^/.]+)$/
 export const extname: typeof path.extname = function (p) {
   const match = _EXTNAME_RE.exec(normalizeWindowsPath(p))
-  return (match && match[0]) || ''
+  return (match && match[1]) || ''
 }
 
 // relative
diff --git a/src/utils.ts b/src/utils.ts
index f0ae8ae..b4920e1 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -17,10 +17,10 @@ export function normalizeAliases (_aliases: Record<string, string>) {
   return aliases
 }
 
-const FILENAME_RE = /(?<=^|[\\/])([^\\/]+?)(?=(\.[^.]+)?$)/
+const FILENAME_RE = /(^|[\\/])([^\\/]+?)(?=(\.[^.]+)?$)/
 
 export function filename (path: string) {
-  return path.match(FILENAME_RE)?.[0]
+  return path.match(FILENAME_RE)?.[2]
 }
 
 // --- internals ---