If you want to catch that specific scenario you could add an additional check. Simply trimming the string before the length check will handle this case.
Running the unit tests against the new code and it only fails on the infinity and boolean literals, and the only time that should be a problem is if you're generating code (really, who would type in a literal and check if it's numeric?
Validation can be defined by many different methods, and deployed in many different ways.
Server side validation is performed by a web server, after input has been sent to the server.
In my opinion, if it's an array then its not numeric. I realize this has been answered many times, but the following is a decent candidate which can be useful in some scenarios.