From f2cfef1ec2c20eb04d327cd70d98bd07f7d72f00 Mon Sep 17 00:00:00 2001
From: Seiya <20365512+seiyab@users.noreply.github.com>
Date: Wed, 10 Dec 2025 00:22:42 +0900
Subject: [PATCH] Correct frontmatter length measurement (#18453)

---
 changelog_unreleased/html/18453.md            | 26 +++++++++++++++++++
 src/main/front-matter/parse.js                |  5 +++-
 .../__snapshots__/format.test.js.snap         | 22 ++++++++++++++++
 tests/format/html/front-matter/unicode.html   |  5 ++++
 .../__snapshots__/format.test.js.snap         | 22 ++++++++++++++++
 tests/format/markdown/front-matter/unicode.md |  5 ++++
 6 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 changelog_unreleased/html/18453.md
 create mode 100644 tests/format/html/front-matter/unicode.html
 create mode 100644 tests/format/markdown/front-matter/unicode.md

diff --git a/changelog_unreleased/html/18453.md b/changelog_unreleased/html/18453.md
new file mode 100644
index 000000000..bb1e4f0d1
--- /dev/null
+++ b/changelog_unreleased/html/18453.md
@@ -0,0 +1,26 @@
+#### Fix a bug around unicode in front matter (#18453 by @seiyab)
+
+<!-- prettier-ignore -->
+```html
+<!-- Input -->
+---
+title: 😄
+---
+
+<!-- comment -->
+
+<!-- Prettier stable -->
+---
+title: 😄
+---
+
+
+<!-- comment --
+
+<!-- Prettier main -->
+---
+title: 😄
+---
+
+<!-- comment -->
+```
diff --git a/src/main/front-matter/parse.js b/src/main/front-matter/parse.js
index 187659371..43203b01d 100644
--- a/src/main/front-matter/parse.js
+++ b/src/main/front-matter/parse.js
@@ -106,7 +106,10 @@ function parse(text) {
     frontMatter,
     get content() {
       const { raw } = frontMatter;
-      return raw.replaceAll(/[^\n]/gu, " ") + text.slice(raw.length);
+      return (
+        // eslint-disable-next-line require-unicode-regexp -- string length matters so intentionally not using unicode flag
+        raw.replaceAll(/[^\n]/g, " ") + text.slice(raw.length)
+      );
     },
   };
 }
diff --git a/tests/format/html/front-matter/__snapshots__/format.test.js.snap b/tests/format/html/front-matter/__snapshots__/format.test.js.snap
index 1387c1c95..36488d2ae 100644
--- a/tests/format/html/front-matter/__snapshots__/format.test.js.snap
+++ b/tests/format/html/front-matter/__snapshots__/format.test.js.snap
@@ -116,3 +116,25 @@ Test <a href="https://prettier.io">abc</a>.
 
 ================================================================================
 `;
+
+exports[`unicode.html format 1`] = `
+====================================options=====================================
+parsers: ["html"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+<!-- this is a comment -->
+
+=====================================output=====================================
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+<!-- this is a comment -->
+
+================================================================================
+`;
diff --git a/tests/format/html/front-matter/unicode.html b/tests/format/html/front-matter/unicode.html
new file mode 100644
index 000000000..015484718
--- /dev/null
+++ b/tests/format/html/front-matter/unicode.html
@@ -0,0 +1,5 @@
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+<!-- this is a comment -->
diff --git a/tests/format/markdown/front-matter/__snapshots__/format.test.js.snap b/tests/format/markdown/front-matter/__snapshots__/format.test.js.snap
index 7434355e8..48af5cff3 100644
--- a/tests/format/markdown/front-matter/__snapshots__/format.test.js.snap
+++ b/tests/format/markdown/front-matter/__snapshots__/format.test.js.snap
@@ -79,3 +79,25 @@ text
 
 ================================================================================
 `;
+
+exports[`unicode.md format 1`] = `
+====================================options=====================================
+parsers: ["markdown"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+## Retrospective
+
+=====================================output=====================================
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+## Retrospective
+
+================================================================================
+`;
diff --git a/tests/format/markdown/front-matter/unicode.md b/tests/format/markdown/front-matter/unicode.md
new file mode 100644
index 000000000..058b9f2f3
--- /dev/null
+++ b/tests/format/markdown/front-matter/unicode.md
@@ -0,0 +1,5 @@
+---
+title: "ABC 漢字 🇯🇵"
+---
+
+## Retrospective