@since Method available since Release 3.6.0
string $expectedRegex:
public function expectOutputRegex($expectedRegex) {
  if ($this->outputExpectedString !== NULL) {
    throw new PHPUnit_Framework_Exception();
  }
  if (is_string($expectedRegex) || is_null($expectedRegex)) {
    $this->outputExpectedRegex = $expectedRegex;
  }
}