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.
oleibman