code: public function apply(Scope $scope, GenericList $arguments) {
  		$nargs = count($arguments);
  		if ($nargs < 3) throw new \BadFunctionCallException("RegexReplaceCallback: Function expects at least 3 parameters.");
  		$args = [];
  		
  		for ($i = 0; $i < $nargs; $i++) {
  			if ($i == 4) break;
  			$args[] = $arguments[$i]->evaluate($scope);
  		}
  		
  		if (!is_callable($args[1])) throw new \InvalidArgumentException("RegexReplaceCallback: Second parameter is not a valid callback.");
  		if ($nargs < 4) return call_user_func_array('preg_replace_callback', $args);
  		else {
  			$target = $arguments[4];
  			if (!($target instanceof Symbol))
  				throw new \InvalidArgumentException(sprintf("RegexReplaceCallback: Expected symbol as fourth parameter but %s was found instead.", substr(strtolower(strstr(get_class($arguments[4]), '\\')), 1)));
  			$ref = $target->symbol;
  			$result = preg_replace_callback($args[0], $args[1], $args[2], $args[3], $count);
  			$scope->symbols[$ref] = $count;
  			return $result;
  		}
  	}
docs: Replaces all matches specified by a pattern on a given subject
  Usage: see RegexPackageTest class
  Returns: array | string
  (non-PHPdoc)
  @see \eMacros\Applicable::apply()