diff --git a/.gitignore b/.gitignore
index f357fd7..12ba1de 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
-/vendor/
+/build/
 /tools/
-/composer.lock
+/vendor/
 /.cs-check.json
-/build
+/.phpunit.result.cache
+/composer.lock
diff --git a/src/DotGit.php b/src/DotGit.php
index e7c45d0..e6811c8 100644
--- a/src/DotGit.php
+++ b/src/DotGit.php
@@ -57,15 +57,17 @@ private function __construct(string $pathToDotGit)
             $this->gitDir = $pathToDotGit;
             return;
         }
-        // additional worktree with a .git file referencing the original .git directory
+        // Additional worktree with a .git file referencing the original .git directory.
         // The referenced path is not required to be named '.git'; a worktree of a
         // bare repository, or of one created with --separate-git-dir, points at a
         // directory with any name. is_dir() below is the actual validation.
+        // This code aligns with the git source code in 'setup.c'.
         if (is_file($pathToDotGit)) {
             $dotGitContent = (string) file_get_contents($pathToDotGit);
-            $match         = [];
-            preg_match('#^gitdir:\s*(?<gitdir>.+)$#m', $dotGitContent, $match);
-            $dir = rtrim($match['gitdir'] ?? '');
+            if (!str_starts_with($dotGitContent, 'gitdir: ')) {
+                throw new RuntimeException('invalid .git file');
+            }
+            $dir = rtrim(substr($dotGitContent, 8), "\r\n");
             if (is_dir($dir)) {
                 $this->gitDir               = $dir;
                 $this->isAdditionalWorktree = true;
diff --git a/tests/unit/DotGitTest.php b/tests/unit/DotGitTest.php
deleted file mode 100644
index fd41748..0000000
--- a/tests/unit/DotGitTest.php
+++ /dev/null
@@ -1,22 +0,0 @@
-<?php
-
-/**
- * This file is part of CaptainHook.
- *
- * (c) Sebastian Feldmann <sf@sebastian-feldmann.info>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-namespace CaptainHook\HookInstaller;
-
-use PHPUnit\Framework\TestCase;
-
-class DotGitTest extends Testcase
-{
-    public function testDummy(): void
-    {
-        $this->assertTrue(true);
-    }
-}
diff --git a/tests/unit/GitDirParseTest.php b/tests/unit/GitDirParseTest.php
new file mode 100644
index 0000000..d2c1c9d
--- /dev/null
+++ b/tests/unit/GitDirParseTest.php
@@ -0,0 +1,80 @@
+<?php
+
+/**
+ * This file is part of CaptainHook.
+ *
+ * (c) Sebastian Feldmann <sf@sebastian-feldmann.info>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CaptainHook\HookInstaller;
+
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+class GitDirParseTest extends TestCase
+{
+    /**
+     * @link https://github.com/captainhook-git/hook-installer/pull/5#discussion_r3729235534
+     */
+    public static function gitDirFormatsInvalidData(): array
+    {
+        return [
+            'no space after colon' => [
+                "gitdir:/foo/.bare\r",
+            ],
+
+            'invalid whitespace after colon' => [
+                "gitdir:\n/foo/.bare\n\n",
+            ],
+
+            'text before gitdir' => [
+                "header\ngitdir: /foo/.bare",
+            ],
+        ];
+    }
+
+    #[DataProvider('gitDirFormatsInvalidData')]
+    public function testParseGitDirInvalid(string $input): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('invalid .git file');
+
+        $path = $this->writeTempDotGit($input);
+
+        DotGit::searchInPath($path);
+    }
+
+    private function writeTempDotGit(string $content): string
+    {
+        $dir = $this->createTempDir();
+
+        if (file_put_contents("$dir/.git", $content) === false) {
+            throw new RuntimeException('Failed to create temporary git path');
+        }
+
+        return $dir;
+    }
+
+    private function createTempDir(): string
+    {
+        $path = tempnam(sys_get_temp_dir(), 'captainhook-hook-installer-');
+
+        if ($path === false) {
+            throw new RuntimeException('Failed to create temporary path');
+        }
+
+        if (unlink($path) === false) {
+            throw new RuntimeException('Failed to delete temporary path');
+        }
+
+        if (mkdir($path, 0700) === false) {
+            throw new RuntimeException('Failed to create temporary directory');
+        }
+
+        return $path;
+    }
+}