Handle Google-only Formulas Exported from Google Sheets

Fix #1637, which went stale but is now re-opened. When Google Sheets exports a document to Xlsx or Ods, it replaces Google-only formulas with something that Excel or LibreOffice can handle. In the test case accompanying this PR, cell C1 on Google Sheets contains `=flatten(A1:A5,B1:B5)`. On export, C1:C10 (the actual result is a 10*1 array) are changed to `=IFERROR(__xludf.DUMMYFUNCTION("flatten(A1:A5, B1:B5)"),1.0)`, where `1.0` is replaced by the calculated value for each cell in question.

The issue reports an Internal Error when evaluating such a formula. I am unable to duplicate that. However, PhpSpreadsheet evaluates the cell as a `#NAME?` error rather than the correct value (1.0 for cell C1). The reason is that `__xludf.DUMMYFUNCTION` does not match the regexp for formulas, but does match the regexp for defined names. Not finding such a defined name results in the Name error. Altering the formula regexp to recognize `__xludf.` is easy, and solves the problem.

Author: oleibman

src/PhpSpreadsheet/Calculation/Calculation.php

@@ -35,7 +35,7 @@ class Calculation extends CalculationLocale
     //    Opening bracket
     const CALCULATION_REGEXP_OPENBRACE = '\(';
     //    Function (allow for the old @ symbol that could be used to prefix a function, but we'll ignore it)
-    const CALCULATION_REGEXP_FUNCTION = '@?(?:_xlfn\.)?(?:_xlws\.)?([\p{L}][\p{L}\p{N}\.]*)[\s]*\(';
+    const CALCULATION_REGEXP_FUNCTION = '@?(?:_xlfn\.)?(?:_xlws\.)?((?:__xludf\.)?[\p{L}][\p{L}\p{N}\.]*)[\s]*\(';
     //    Cell reference (cell or range of cells, with or without a sheet reference)
     const CALCULATION_REGEXP_CELLREF = '((([^\s,!&%^\/\*\+<>=:`-]*)|(\'(?:[^\']|\'[^!])+?\')|(\"(?:[^\"]|\"[^!])+?\"))!)?\$?\b([a-z]{1,3})\$?(\d{1,7})(?![\w.])';
     // Used only to detect spill operator #

tests/PhpSpreadsheetTests/Reader/Xlsx/Issue1637Test.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Reader\Xlsx;
+
+use PhpOffice\PhpSpreadsheet\Reader\Xlsx;
+use PHPUnit\Framework\TestCase;
+
+class Issue1637Test extends TestCase
+{
+    private static string $testbook = 'tests/data/Reader/XLSX/issue.1637.xlsx';
+
+    public function testXludf(): void
+    {
+        $reader = new Xlsx();
+        $spreadsheet = $reader->load(self::$testbook);
+        $sheet = $spreadsheet->getActiveSheet();
+        self::assertSame(
+            '=IFERROR(__xludf.DUMMYFUNCTION("flatten(A1:A5, B1:B5)"),1.0)',
+            $sheet->getCell('C1')->getValue()
+        );
+        self::assertSame(1.0, $sheet->getCell('C1')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+}