From 8d67a099571ec4b00b3d3929e2fadad2e4d0791f Mon Sep 17 00:00:00 2001
From: kdelay <kdelay20@gmail.com>
Date: Fri, 4 Sep 2026 00:01:16 +0900
Subject: [PATCH] fix: accept a negative `a` in `:nth-child(an+b)` and an
 uppercase `n`

`REGEX_NTH_OF` only allowed a `+` offset after a negative `a`, only an
unsigned lone `<integer>`, and a lowercase `n`, so valid `<an+b>` values
such as `-1`, `-2n`, `-2n-1` and `2N+1` failed to compile with
`css_expected_identifier`. Fold the two sign branches into one and match
`n` case-insensitively. The `of` keyword is left as it was.
---
 .changeset/signed-nth-child.md                |  5 ++++
 .../src/compiler/phases/1-parse/read/style.js |  5 ++--
 .../nth-child-signed-an-plus-b/expected.css   | 17 ++++++++++++++
 .../nth-child-signed-an-plus-b/input.svelte   | 23 +++++++++++++++++++
 4 files changed, 48 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/signed-nth-child.md
 create mode 100644 packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/expected.css
 create mode 100644 packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/input.svelte

diff --git a/.changeset/signed-nth-child.md b/.changeset/signed-nth-child.md
new file mode 100644
index 0000000000..ee6144a5ac
--- /dev/null
+++ b/.changeset/signed-nth-child.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: accept a negative `a` in `:nth-child(an+b)` and an uppercase `n`
diff --git a/packages/svelte/src/compiler/phases/1-parse/read/style.js b/packages/svelte/src/compiler/phases/1-parse/read/style.js
index 574c353adb..126173dbd5 100644
--- a/packages/svelte/src/compiler/phases/1-parse/read/style.js
+++ b/packages/svelte/src/compiler/phases/1-parse/read/style.js
@@ -9,9 +9,10 @@ const REGEX_COMBINATOR = /(\+|~|>|\|\|)/y;
 const REGEX_PERCENTAGE = /\d+(\.\d+)?%/y;
 // `of` must be preceded by whitespace, otherwise it would be part of the `<an+b>` token
 // (`2nof` is a single dimension token). It does not need to be followed by whitespace,
-// because a `.`, `#`, `[`, `*`, `:` or `&` already ends the `of` identifier — minifiers rely on that
+// because a `.`, `#`, `[`, `*`, `:` or `&` already ends the `of` identifier — minifiers rely on that.
+// The sign of `a` (or of a lone `<integer>`) may be `+` or `-`, and the `n` is case-insensitive
 const REGEX_NTH_OF =
-	/(even|odd|\+?(\d+|\d*n(\s*[+-]\s*\d+)?)|-\d*n(\s*\+\s*\d+))((?=\s*[,)])|\s+of(\s+|(?=[.#[*:&])))/y;
+	/(even|odd|[+-]?(\d+|\d*[nN](\s*[+-]\s*\d+)?))((?=\s*[,)])|\s+of(\s+|(?=[.#[*:&])))/y;
 const REGEX_WHITESPACE_OR_COLON = /[\s:]/;
 const REGEX_LEADING_HYPHEN_OR_DIGIT = /-?\d/y;
 const REGEX_VALID_IDENTIFIER_CHAR = /[a-zA-Z0-9_-]/;
diff --git a/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/expected.css b/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/expected.css
new file mode 100644
index 0000000000..ed8edc4656
--- /dev/null
+++ b/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/expected.css
@@ -0,0 +1,17 @@
+
+	/* the `a` of `an+b`, and a lone `<integer>`, can be negative, and `n` is case-insensitive */
+	li.svelte-xyz:nth-child(-1) {
+		color: red;
+	}
+	li.svelte-xyz:nth-child(-2n) {
+		color: green;
+	}
+	li.svelte-xyz:nth-child(-2n-1) {
+		color: blue;
+	}
+	li.svelte-xyz:nth-child(2N+1) {
+		color: yellow;
+	}
+	li.svelte-xyz:nth-child(-2n-1 of .important) {
+		color: purple;
+	}
diff --git a/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/input.svelte b/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/input.svelte
new file mode 100644
index 0000000000..f92cdaa222
--- /dev/null
+++ b/packages/svelte/tests/css/samples/nth-child-signed-an-plus-b/input.svelte
@@ -0,0 +1,23 @@
+<ul>
+	<li>a</li>
+	<li class="important">b</li>
+</ul>
+
+<style>
+	/* the `a` of `an+b`, and a lone `<integer>`, can be negative, and `n` is case-insensitive */
+	li:nth-child(-1) {
+		color: red;
+	}
+	li:nth-child(-2n) {
+		color: green;
+	}
+	li:nth-child(-2n-1) {
+		color: blue;
+	}
+	li:nth-child(2N+1) {
+		color: yellow;
+	}
+	li:nth-child(-2n-1 of .important) {
+		color: purple;
+	}
+</style>