From b11676a0bc3549e5a81c94314d9e15d27efa4612 Mon Sep 17 00:00:00 2001
From: Felix Boehm <188768+fb55@users.noreply.github.com>
Date: Sat, 3 Oct 2026 23:14:32 +0100
Subject: [PATCH] fix(pseudos): avoid overlapping whitespace in nth parsing

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 src/pseudo-selectors/filters.ts | 2 +-
 test/pseudo-classes.ts          | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/pseudo-selectors/filters.ts b/src/pseudo-selectors/filters.ts
index 83f0a69d..ea98cd98 100644
--- a/src/pseudo-selectors/filters.ts
+++ b/src/pseudo-selectors/filters.ts
@@ -32,7 +32,7 @@ function extendedFilter(tag: string[], range: string[]): boolean {
 }
 
 /** @see {@link https://www.w3.org/TR/selectors-4/#the-nth-child-pseudo} */
-const nthOfRegex = /^(.+?)\s+of\s+(.+)$/is;
+const nthOfRegex = /^(.+?\s)of\s+(.+)$/is;
 
 /** A pre-compiled pseudo filter. */
 export type Filter = <Node, ElementNode extends Node>(
diff --git a/test/pseudo-classes.ts b/test/pseudo-classes.ts
index 92a58545..1feeb8ce 100644
--- a/test/pseudo-classes.ts
+++ b/test/pseudo-classes.ts
@@ -9,6 +9,13 @@ const dom = parseDOM(
     "<div><p>In the end, it doesn't really Matter.</p><div>Indeed-that's a delicate matter.</div>",
 ) as Element[];
 
+// The previous pattern backtracked on long whitespace, taking minutes here.
+it("should reject a long whitespace nth argument without backtracking", () => {
+    expect(() =>
+        CSSselect.compile(`:nth-child(${" ".repeat(350_000)})`),
+    ).toThrow("n-th rule couldn't be parsed");
+}, 2000);
+
 describe(":icontains", () => {
     describe("ignore case", () => {
         it("should match full string", () => {