``[A-Za-z0-9_]`` is not the regex defining allowed chars. I can use ``§`` in a function name if I want (Twig [does it](https://github.com/fabpot/Twig/blob/master/test/Twig/Tests/IntegrationTest.php#L160) for testing purpose).

Valid chars in PHP identifiers (be it function names or variables) are matched by ``[a-zA-Z0-9_\x7f-\xff]`` (the full regex taking care of forbiddign numbers as the first char is available in the [PHP documentation](http://www.php.net/manual/en/functions.user-defined.php))