String starting with \ is valid class name

src/Reflection/ClassNameHelper.php

@@ -10,7 +10,7 @@ class ClassNameHelper
 	public static function isValidClassName(string $name): bool
 	{
 		// from https://stackoverflow.com/questions/3195614/validate-class-method-names-with-regex#comment104531582_12011255
-		return Strings::match($name, '/^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*(\\\\[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*)*$/') !== null;
+		return Strings::match(ltrim($name, '\\'), '/^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*(\\\\[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*)*$/') !== null;
 	}
 
 }

tests/PHPStan/Rules/Classes/InstantiationRuleTest.php

@@ -342,4 +342,9 @@ public function testBug3425(): void
 		]);
 	}
 
+	public function testBug5002(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-5002.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Classes/data/bug-5002.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Test;
+
+class Foo
+{
+
+}
+
+function (): void {
+	$name = '\Test\Foo';
+	new $name();
+};
+
+function (): void {
+	$name = '\\Test\Foo';
+	new $name();
+};