Allow underscore in extended static files in theme

dev/tests/integration/testsuite/Magento/Framework/App/Utility/FilesTest.php

@@ -6,7 +6,6 @@
 
 namespace Magento\Framework\App\Utility;
 
-use Magento\Framework\App\Utility\Files;
 use Magento\Framework\Component\ComponentRegistrar;
 
 class FilesTest extends \PHPUnit\Framework\TestCase
@@ -159,4 +158,24 @@ public function testReadLists()
         }
         $this->assertSame($expected, $actual);
     }
+
+    /**
+     * Tests if underscores are allowed in extended static files in theme.
+     *
+     * @magentoComponentsDir Magento/Framework/App/Utility/_files/design
+     * @magentoAppIsolation enabled
+     * @magentoDbIsolation enabled
+     */
+    public function testAllowUnderscoreInExtendedFiles()
+    {
+        $this->assertNotEmpty(
+            array_filter(
+                $this->model->getStaticPreProcessingFiles('*.less'),
+                function ($resource) {
+                    return $resource[3] === 'Module_Third_Party'
+                        && $resource[4] === 'css/source/_module.less';
+                }
+            )
+        );
+    }
 }

dev/tests/integration/testsuite/Magento/Framework/App/Utility/_files/design/frontend/Test/default/Module_Third_Party/web/css/source/_module.less

@@ -0,0 +1,8 @@
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+ .a-random-css-rule {
+     background: red;
+ }
+

dev/tests/integration/testsuite/Magento/Framework/App/Utility/_files/design/frontend/Test/default/registration.php

@@ -0,0 +1,9 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+use Magento\Framework\Component\ComponentRegistrar;
+
+ComponentRegistrar::register(ComponentRegistrar::THEME, 'frontend/FrameworkUtilityTest/default', __DIR__);

dev/tests/integration/testsuite/Magento/Framework/App/Utility/_files/design/frontend/Test/default/theme.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
+    <title>Test Default</title>
+</theme>

lib/internal/Magento/Framework/App/Utility/Files.php

@@ -936,7 +936,7 @@ private function accumulateThemeStaticFiles($area, $locale, $filePattern, &$resu
                 ];
                 $this->_accumulateFilesByPatterns($paths, $filePattern, $files);
                 $regex = '#^' . $themePath .
-                    '/((?P<module>[a-z\d]+_[a-z\d]+)/)?web/(i18n/(?P<locale>[a-z_]+)/)?(?P<path>.+)$#i';
+                    '/((?P<module>[a-z\d]+_[a-z_\d]+)/)?web/(i18n/(?P<locale>[a-z_]+)/)?(?P<path>.+)$#i';
                 foreach ($files as $file) {
                     if (preg_match($regex, $file, $matches)) {
                         $result[] = [