fix: correctly parse escaped unicode characters in css selector

.changeset/lazy-singers-pretend.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly parse escaped unicode characters in css selector

packages/svelte/src/compiler/phases/1-parse/read/style.js

@@ -12,6 +12,7 @@ const REGEX_NTH_OF =
 const REGEX_WHITESPACE_OR_COLON = /[\s:]/;
 const REGEX_LEADING_HYPHEN_OR_DIGIT = /-?\d/;
 const REGEX_VALID_IDENTIFIER_CHAR = /[a-zA-Z0-9_-]/;
+const REGEX_UNICODE_SEQUENCE = /^\\[0-9a-fA-F]{1,6}(\r\n|\s)?/;
 const REGEX_COMMENT_CLOSE = /\*\//;
 const REGEX_HTML_COMMENT_CLOSE = /-->/;
 
@@ -580,25 +581,26 @@ function read_identifier(parser) {
 		e.css_expected_identifier(start);
 	}
 
-	let escaped = false;
-
 	while (parser.index < parser.template.length) {
 		const char = parser.template[parser.index];
-		if (escaped) {
-			identifier += '\\' + char;
-			escaped = false;
-		} else if (char === '\\') {
-			escaped = true;
+		if (char === '\\') {
+			const sequence = parser.match_regex(REGEX_UNICODE_SEQUENCE);
+			if (sequence) {
+				identifier += String.fromCodePoint(parseInt(sequence.slice(1), 16));
+				parser.index += sequence.length;
+			} else {
+				identifier += '\\' + parser.template[parser.index + 1];
+				parser.index += 2;
+			}
 		} else if (
 			/** @type {number} */ (char.codePointAt(0)) >= 160 ||
 			REGEX_VALID_IDENTIFIER_CHAR.test(char)
 		) {
 			identifier += char;
+			parser.index++;
 		} else {
 			break;
 		}
-
-		parser.index++;
 	}
 
 	if (identifier === '') {

packages/svelte/tests/css/samples/unicode-identifier/_config.js

@@ -0,0 +1,76 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".\\61  sdf"',
+			start: {
+				line: 22,
+				column: 1,
+				character: 465
+			},
+			end: {
+				line: 22,
+				column: 10,
+				character: 474
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".\\61\n\tsdf"',
+			start: {
+				line: 23,
+				column: 1,
+				character: 492
+			},
+			end: {
+				line: 24,
+				column: 4,
+				character: 501
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".\\61\n sdf"',
+			start: {
+				line: 25,
+				column: 1,
+				character: 519
+			},
+			end: {
+				line: 26,
+				column: 4,
+				character: 528
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector "#\\31span"',
+			start: {
+				line: 28,
+				column: 1,
+				character: 547
+			},
+			end: {
+				line: 28,
+				column: 9,
+				character: 555
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector "#\\31 span"',
+			start: {
+				line: 29,
+				column: 1,
+				character: 573
+			},
+			end: {
+				line: 29,
+				column: 10,
+				character: 582
+			}
+		}
+	]
+});

packages/svelte/tests/css/samples/unicode-identifier/expected.css

@@ -0,0 +1,21 @@
+	#\31\32\33 .svelte-xyz{ color: green; }
+	#\31 23.svelte-xyz { color: green; }
+	#line\a break.svelte-xyz { color: green; }
+	#line\a
+break.svelte-xyz { color: green; }
+	#line\00000abreak.svelte-xyz { color: green; }
+	#line\00000a break.svelte-xyz { color: green; }
+	#line\00000a	break.svelte-xyz { color: green; }
+	.a\1f642 b.svelte-xyz { color: green; }
+
+	.\61sdf.svelte-xyz { color: green; }
+
+	/* (unused) .\61  sdf { color: red; }*/
+	/* (unused) .\61
+	sdf { color: red; }*/
+	/* (unused) .\61
+ sdf { color: red; }*/
+
+	/* (unused) #\31span { color: red; }*/
+	/* (unused) #\31 span { color: red; }*/
+	#\31 .svelte-xyz span:where(.svelte-xyz) { color: green; }

packages/svelte/tests/css/samples/unicode-identifier/expected.html

@@ -0,0 +1,7 @@
+<div id="123" class="svelte-xyz"></div>
+<div class="svelte-xyz" id="line
+break"></div>
+<div class="a🙂b svelte-xyz"></div>
+<div class="asdf svelte-xyz"></div>
+<div class="&#97;sdf svelte-xyz"></div>
+<div id="1" class="svelte-xyz"><span class="svelte-xyz"></span></div>

packages/svelte/tests/css/samples/unicode-identifier/input.svelte

@@ -0,0 +1,31 @@
+<div id="123"></div>
+<div id="line
+break"></div>
+<div class="a🙂b"></div>
+<div class="asdf"></div>
+<div class="&#97;sdf"></div>
+<div id="1"><span></span></div>
+
+<style>
+	#\31\32\33 { color: green; }
+	#\31 23 { color: green; }
+	#line\a break { color: green; }
+	#line\a
+break { color: green; }
+	#line\00000abreak { color: green; }
+	#line\00000a break { color: green; }
+	#line\00000a	break { color: green; }
+	.a\1f642 b { color: green; }
+
+	.\61sdf { color: green; }
+
+	.\61  sdf { color: red; }
+	.\61
+	sdf { color: red; }
+	.\61
+ sdf { color: red; }
+
+	#\31span { color: red; }
+	#\31 span { color: red; }
+	#\31  span { color: green; }
+</style>