Well, since there is no "s" modifier on the regex, it is at least testing making sure that the value of $command_str does not contain "\n", which might be worth something, depending on how this variable is used later on.

But if the whole script were in my hands, the next thing I would do would be to search for occurrences/uses of $command_str outside (following) that if block, because any such occurrence/usage could be getting a still-tainted value (in the case where that "if" condition wasn't met).

(Update: and then, of course, I would also look very carefully at how this variable is being used inside that "if" block.)